"We were having a couple of beers and a chat about the trip to Afghanistan," says Seven Network political editor Mark Riley, "and he made a couple of remarks that she thought were just a little bit out of school." How did Credlin signal to Abbott that he should watch his words? "She gave him a swift kick in the shin." Riley pauses. "It was done with good humour. But for us, it was a really interesting insight into their relationship."

Credlin, 43, is the most discussed woman in Canberra, and probably the most powerful. She heads the prime minister's office, effectively the command centre of the Liberal-National Coalition government, and is widely believed to have more influence over the way the country is run than most of our elected representatives. (She has been known to refer to parliamentarians as "the front men".) The key to her clout is the nature of her alliance with Abbott. "He relies on her heavily and respects her judgment," says a senior Liberal. "She is the person in politics he's closest to, no doubt about that."

Credlin is a physically imposing figure: 183 centimetres tall, with long, dark hair, a taste for leopard print and a collection of elegant size-11 stilettos. The term "glamazon" might have been invented for her. And that isn't the only significant difference between her and the chiefs of staff of previous prime ministers. Traditionally, they have been back-room people, operating quietly behind closed doors. For Credlin, "one has to construct a new paradigm", says Stephen Brady, official secretary to the governor-general, who has had semi-regular dealings with her and several of her predecessors. "It's a different type of authority. It's more overt."

Brady, a career diplomat, suggests Credlin is best compared with a US president's chief of staff, "who has cabinet-level status, and is not just gatekeeper and policy adviser but the voice, in a way, of the president. When you are speaking with Peta, you know that she's speaking, really, on behalf of the prime minister."

Unlike desk-bound chiefs of staff of the past, Credlin is at the PM's side at many official events. Internet entrepreneur Daniel Petre tells of a dinner at which "a person across the table asked a question of Tony Abbott and she jumped in with the answer. Not only did he let her finish the answer, he didn't actually say anything." I relay the anecdote to a former senior Liberal Party official, who replies: "I've seen that on a number of occasions. And it makes me very, very uncomfortable."

Credlin is married to Brian Loughnane, the Liberal Party's federal director, which makes the former party official even uneasier. "There is an unwritten rule that the party keeps an eye on the prime minister's office, and the prime minister's office keeps an eye on the party," he says. "It makes it pretty much impossible for that to happen when you have this watertight connection between the federal director and the prime minister's chief of staff."

Each parliamentary sitting day, Abbott has a morning meeting with his seven most senior ministers. Loughnane and Credlin attend, too. Attorney-General George Brandis says Credlin is an astute political tactician who "sees the game from every point on the field. She's the person who will say, 'Look, don't do that, because of this risk or that.' She's a tremendous, pragmatic political person but she is also completely in command of the policy side of things." Agriculture Minister Barnaby Joyce says Credlin "dispels the myth that women don't have a strong role in the management group of our nation. Believe you me, she has a very strong role."

Since the Coalition won office last year, Credlin has overseen the vetting - and in many cases, vetoing - of all staff appointments by Liberal and National parliamentarians. Even cabinet ministers have had hiring decisions overruled. For instance, Employment Minister Eric Abetz, the leader of the government in the Senate, had his choice of chief of staff knocked back. "Her enemies would probably call her a control freak," another cabinet member says of Credlin, referring to the perception that she closely monitors every aspect, large and small, of the new regime in the national capital. Not only is she rumoured to have been instrumental in the ousting of several senior bureaucrats, she chose the fabric for the couches in the prime minister's Parliament House sitting room.

"The media and the public like to pigeonhole," says her friend, federal Education Minister Christopher Pyne. "And the pigeonhole for Peta is 'scary, driven career woman who you shouldn't cross or you'll end up at the bottom of Sydney Harbour with cement shoes'. "

There is certainly plenty of fear and resentment of Credlin, most of it within Coalition ranks. Queensland Liberal senator Ian Macdonald struck a chord with many of his colleagues when he complained in parliament last December that the PM's office, led by Credlin, had a culture of "almost obsessive centralised control". Joyce, who describes Credlin as "a fanatic for detail", says: "She can be very tough. Which, er, some find off-putting."

One Sunday, Credlin and I sit down together in her Parliament House office - a large, pleasant space decorated with Aboriginal art from her own collection. A bike leans against one wall. A television with the sound turned down is tuned to a news channel. Credlin has reluctantly agreed to the meeting after making clear that she is unhappy about being profiled. ("People think I court this sort of stuff but I don't. I'm horrified by it.") For the sake of accuracy, she will provide biographical information. "This is not an interview," she says firmly.

At one point, Abbott pops in from his adjoining office and I ask if he would consider making some comments for the story. At least, that's what I intend to do. For some reason, I find myself directing the question to Credlin and referring to the prime minister in the third person.

As Abbott retreats through the connecting door, he says the prime minister will think about it. "The prime minister has a mind of his own," he adds.

At Question Time in the House of Representatives on an autumn afternoon, Abbott repeatedly leaves his seat and walks to the adviser's box to talk quietly to Credlin. Five times in the space of an hour, he ducks over to confer with her. Between his visits, she dashes off notes, which a uniformed clerk delivers to various Coalition frontbenchers.

So involved is she in the proceedings that I can see how she could once momentarily have forgotten that only actual members of parliament are permitted to interject in debate in the chamber. During a speech in Parliament in 2012 by the then prime minister, Julia Gillard, Credlin said: "Such a liar." Then, when Labor's Anthony Albanese got to his feet to protest, she said something like, "Sit down, you idiot." She has always maintained in her defence that the remarks were so quiet they were practically under her breath, but the then speaker, Peter Slipper, warned that he would ban her from the House if she did it again.

Normally, it is Credlin who hands out admonishments. In the most entertaining TV footage from the election campaign, shot by a Seven news crew during a visit by Abbott to the Holsworthy army base in south-western Sydney, she was seen jabbing her finger at the chest of the Liberal member for Fadden, Stuart Robert. Seven's Mark Riley, who learnt she was berating Robert for pestering Abbott to sign a neck tie, says he was slightly apprehensive about how she would react to the screening of the clip. "But the next day she sought me out and said she enjoyed it," he says. "She thought it was good because it sent the right message." And that message was? "You don't cross Credlin."

George Brandis, who wrote her a character reference after she was charged with drink-driving last May (she pleaded guilty to being at the wheel with a blood-alcohol level of 0.075 - no conviction was recorded), says: "People who know her less well than I do might think she's a tyrant ... I think she's a forceful person." What Coalition MPs and senators know for sure is that to get on Credlin's wrong side is a bad career move. "If she thinks somebody is a dud, and not worthy of higher political honours, she will make that view known," says a senior Liberal. "And it would be surprising if that view didn't prevail. She has a lot of power." (When I ask Robert, now assistant defence minister, if he has any hard feelings about the dressing-down Credlin gave him, he says jovially, "Oh goodness, no! Peta and I are mates.")

Her management style has always been bracingly direct. One of her past employers, former senator Helen Coonan, confirms that Credlin sent "some pretty savage emails" while heading her office. "If, in her eyes, you don't cut it, you won't be left wondering," Coonan says. "But I don't think she's harsh or unreasonable in her judgments." In any case, "it's up to the chief of staff to put a bit of stick about. That's her role."

Political strategist Bruce Hawker says he sympathises with Credlin, knowing from his own experience as chief of staff to former NSW Labor premier Bob Carr that the role requires someone who is "about 70 per cent diplomat, 30 per cent thug ... There are a lot of people who, left to themselves, will only do damage to themselves and the government. You do need a sergeant-major there to go out and haul them in, regardless of what their rank is."

Hawker says Abbott himself used to be one of those MPs who give minders nightmares by failing to stick to the party line. "He had a propensity to say basically what came into his head. But he was very disciplined through his time as opposition leader. Credlin did a remarkable job of keeping him on a very tight rein."

Treasurer Joe Hockey cautions against overstating Credlin's sway over Abbott. "She's enormously influential - there's no argument about that," Hockey says. "But there's no suggestion at all that she's pulling the leash. That might be the perception, but it's not the reality." Environment Minister Greg Hunt says Credlin and Abbott make an excellent team: "He is his own man. He just happens to have a world-class chief of staff. It's a very healthy relationship."

I ask Abbott by email whether it is true, as I have heard, that he sometimes refers to Credlin as "the boss". He replies that he mostly just uses her first name. Occasionally, he says, he calls her "the chief". But "there is only one 'boss' in my office, and that's me".

If you're not in trouble about anything, Credlin is warm and engaging company. "Likeable, but with a hint of ferocity below the surface," is the way one cabinet minister sums her up. A member of the Canberra press gallery says Credlin never really lets down her guard - even when she appears to be confiding in you, she's spruiking for the Coalition cause: "She tells you all these great bits of gossip that are totally advantageous to her side, should you ever decide to pursue them as stories."

Credlin's dirt file is legendary, and she has a reputation as such a skilled player of political hardball that strategic leaks of information damaging to Labor tend to be attributed to her even when she had nothing to do with them. Last October, for instance, a few hours after Labor frontbencher Mark Dreyfus accused Coalition ministers of rorting the parliamentary expenses system, a journalist got a tip-off that Dreyfus had been paid expenses for a family skiing weekend in August 2011. Dreyfus's media adviser, Annie Williams, tells me he had been unaware that his office had mistakenly made the claim, and immediately repaid the money. Still, the resulting news stories were embarrassing and took the sting out of Labor's attack on the government.

Credlin was presumed to be responsible. "What was suggested to us was that it was Peta Credlin who saw him skiing," Williams says, "and that she then stored it away for a year and a half. Terrifying!" But Credlin says in a text message that she can't recall ever seeing a Labor politician on the slopes: "So, not me."

To become the accomplished skier that she is, Credlin had to conquer an extreme fear of heights (she still can't look down from chairlifts). Challenging herself is in her nature: at school in Wycheproof, a rural community in north-western Victoria, she took up debating for the very reason that it made her nervous. She was quite a polished public speaker by the time her family moved to the coastal township of St Leonards, where her parents owned small businesses, including a newsagency and grocery store.

Credlin, aged 15, enrolled at Sacred Heart College in nearby Geelong. The current principal, Anna Negro, who was one of her teachers, was struck by her ability and ambition: "I can remember her saying to me, 'I'm going to be a High Court judge.' "

Politics attracted Credlin, too, and in 1998, armed with a Melbourne University law degree, she joined the staff of the then Victorian Liberal senator Kay Patterson. "She was incredibly good to work with," says Patterson, who in 2000 cut out an article about Brian Loughnane, "the 42-year-old bachelor" who had been appointed director of the party's Victorian branch. "I stuck it on her computer and wrote, 'You need to marry this man,' " Patterson says. "And she did."

The day before the 2002 wedding in Melbourne's St Patrick's Cathedral, John Howard offered Loughnane the federal directorship. Credlin, who had moved from Patterson's office to work for senator Richard Alston, spent three years as Racing Victoria's communications and public affairs manager before returning to Canberra as a senior adviser to defence minister Robert Hill. Barnaby Joyce recalls having shouting matches with her about the sale of Telstra after she was appointed chief of staff to then communications minister, Helen Coonan, in 2006. "I find her a person you can have a big argument with and then forget about it and move on," Joyce says.

Like many political staffers, Credlin has always worked insanely long hours. "We had a karaoke machine in the office, and at two in the morning it sometimes used to get a bit rowdy," Coonan says. "We had to entertain ourselves somehow." I ask Coonan what Credlin's singing voice is like. "Deep and commanding," she replies.

When the Howard government was defeated in 2007, Credlin decided it was time to get out of politics and lead a more normal life. Brendan Nelson, who became Coalition leader, understood why she wanted a break. "Whatever hours you see a minister or prime minister working, increase it by 30 per cent for the chief of staff," he says. Nevertheless, Nelson was able to persuade Credlin to sign on as his social policy adviser, and when he was toppled by Malcolm Turnbull less than a year later, she moved to Turnbull's office as chief of staff. In the only backward move in her career, Credlin was demoted to deputy when Turnbull brought in Chris Kenny, now a columnist at The Australian. "I know she was deeply hurt," says Nelson, now the director of the Australian War Memorial in Canberra. "But she just got on with it."

Abbott won the Liberal leadership by one vote in December 2009. Credlin and many others on the conservative side of politics prepared themselves for a long stretch in opposition: the "Mad Monk", as Abbott was known, was considered too right-wing, too Catholic, altogether too extreme, to appeal to mainstream Australia. When Abbott asked Credlin to be his chief of staff, she cried. She was almost 39, and keen to have a baby. She had lined up a job with the law firm Corrs Chambers Westgarth. She had already given the Liberal Party some of the best years of her life, she protested.

But again she allowed herself to be talked into staying, and she and Abbott worked so well together that the following year the Coalition came within a whisker of being returned to office. "We were at first a disorganised rabble in opposition," says Joe Hockey, "and then became a formidable, united team. She deserves much of the credit for that."

On a flight from Afghanistan to Australia after the 2010 election, Credlin and Abbott had what must have been a tricky conversation. As Howard's health minister, Abbott had tried but failed to limit publicly subsidised in vitro fertilisation (IVF) treatment to three cycles a year for women up to the age of 42, or three in total for older women. Later, he had said the proposal "fell foul of the 'I'm over 40 and I need a baby' brigade".

Now Credlin told him that she intended to embark on an IVF course. She thought perhaps she should resign, but Abbott, who knew that with her help he could become prime minister at the next election, said his attitude to IVF had been widely misunderstood. Not only did he wholeheartedly support her decision to have treatment, he said, she could keep her drugs in his office fridge and inject them in his bathroom.

For almost a year, Credlin had monthly operations requiring general anaesthetic. She would go straight from hospital to Parliament House and be at her desk before lunchtime. On one such day in October 2012, she learnt when she arrived at work that a comedian at a union dinner attended by Labor ministers in the Great Hall the previous night had told a smutty joke suggesting she and Abbott were having an affair. "Everyone was sort of sniggering in the corridors about it," says former Abbott media adviser, Jude Donnelly.

Earlier that week, Julia Gillard had made a widely publicised speech accusing Abbott of sexism and misogyny. Credlin counterpunched by breaking her no-interviews rule, saying in magazine and newspaper articles how hurt she had been by the joke, and how supportive Abbott had been of her and Loughnane's efforts to conceive.

In the lead-up to last September's federal election, Credlin stopped her IVF treatment, deciding that she could not combine it with the rigours of campaigning. Friends say that when she talks about this, her eyes fill with tears. "I was very close to her through that period," says Christopher Pyne. "I think she did put winning, for the greater good of the country, ahead of her own needs."

In Abbott's victory speech, which he delivered surrounded by his wife and three daughters, he called Credlin "the smartest and the fiercest political warrior" in the party.

Credlin's father, who died of a stroke aged 60, was the sort of Catholic who carried rosary beads in his pocket and never missed Mass. Some believe this is one of the reasons that Credlin, who is far less devout, gets Abbott. "She has a sixth sense about the bloke - a great intuition about what he wants to do and how he's going to do it," says Victorian MP Josh Frydenberg, the prime minister's parliamentary secretary. As Abbott puts it in his email to me: "Peta has a very keen sense of what I am likely to think about issues that are coming up."

To Foreign Minister Julie Bishop, the only woman in cabinet, Credlin's deep understanding of Abbott makes her a valuable sounding board. "I'll often run ideas past her to gauge a sense of what the prime minister might think about something," Bishop says. "I'll give her a call and say, 'Peta, what do you think about this?' "

Bishop isn't the only one. Stephen Galilee, a former adviser to Abbott, says: "I know there's been talk about her having too much power and control in government, but the flip side of that is that people who want answers, who want to know their message is going to be communicated to the prime minister, know that they can talk to her and she has his ear." The chief executive of the NSW Minerals Council, Galilee adds that scrutiny is one thing, but "some of the commentary about Peta has an element to it that is a bit malicious and cruel".

Abbott contends she has been judged differently because she is female: "Peta's one of the brightest, most determined and brutally effective people I know," he says. "We call those qualities 'leadership' in a man and if some people regard them as 'scary' in a woman, that's a reflection on them, not Peta." When former governor-general Quentin Bryce hears this story is in the works, she offers the observation: "One has to be careful not to see as negatives features in a woman that would be seen as strengths in a man."

Credlin is regarded as less of an ideologue than Abbott. "I think she's a moderating influence in the office," says independent Tasmanian MP Andrew Wilkie, who has had many discussions with the two of them. "She's intelligent and reasonable. She strikes me as someone who has got quite a big heart."

Publisher Louise Adler says she liked the way Credlin and Abbott interacted at a series of meetings she had with them before the release last year of a revised edition of Abbott's 2009 book, Battlelines. Abbott thought nothing of arriving at a restaurant carrying Credlin's gym bag, for instance. In conversation, he was quite prepared to defer to her. Adler remembers one business lunch at which talk turned to art. "He said something like, 'Art should be about beauty.' And Peta said, 'Don't be silly, Tony. Think about modernism. It's about challenging ideas.' Then she gave him a concise lecture on the history of modernist art."

Theories abound about the inner dynamics of the relationship. "It's my observation that Tony Abbott is much more influenced by women than by men," says George Brandis. "Much more solicitous of their opinions. And, I think, just more comfortable around them."

One well-placed source believes Credlin has a hold over Abbott because he feels grateful to her for steering him into office and guilty about the sacrifices she has made for him. A Liberal parliamentarian maintains that Abbott is calculatedly using Credlin - allowing her to do his dirty work so that he appears statesmanlike and above the fray. "People think he's just some blustering Captain Catholic," this Liberal says, "but Tony is very Machiavellian."

Credlin reminds Louise Adler of C. J. Cregg, the lanky White House press secretary who becomes the president's chief of staff in the sixth series of The West Wing. Credlin's own favourite American political drama is the altogether darker House of Cards. She is addicted to it, apparently. The promotional line for the series: "Behind every great man is a woman with blood on her hands."

Lead-in photo by Rohan Thomson.

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