Witness accounts from inside Australia's detention centres are rare. Walled in behind government secrecy, contracts which bind them to silence, and fear for their future livelihoods, staff and former employees of the organisations running the centres bite their tongues, or confide only in close colleagues, family members or friends.

The few who have spoken to the media have mostly done so anonymously, or through third parties.

Now, the first of what could be a steady trickle of embarrassing whistleblower accounts has emerged in the form of an explosive book, The Undesirables, by a former Salvation Army employee, 26-year-old Sydneysider Mark Isaacs. The title is taken from a term Isaacs says a government staffer was overheard using to describe the asylum-seekers at the camp.

Isaacs was only 24 when, on the strength of a single phone call and with no experience, he was hired by the Salvos and dispatched to Nauru with less than a week's notice to ''provide support'' to asylum seekers detained there.

The date was October 1, 2012, just a fortnight after then prime minister Julia Gillard had reopened the offshore camp in a desperate revival of former prime minister John Howard's ''Pacific solution'' - an attempt to deter asylum seekers by shipping them to the tiny Pacific nation for indefinite detention.

Isaacs completed five rotations through the camp, each of several weeks. The last he saw of ''Topside'', as the internment camp on Nauru was known, was in June last year, shortly before inmates burnt much of it to the ground.

There is a rawness and an immediacy to this account because it is so recent. Originally slated for publication next month, the book - available from Monday - has been rushed out by publisher Hardie Grant Books after the chaotic events at the Manus Island detention centre last month which left one man dead, and scores injured.

Isaacs' book constitutes a warning that, no matter how much physical facilities on Manus and Nauru might improve, it may be impossible to avoid violent periodic eruptions inside the camps in the future.

It is because the men have no hope, he argues, and therefore little to lose.

''Criminals were given a sentence to serve: these men were not even given that,'' Isaacs writes. ''They feared they would die in Nauru, that they would be forgotten, that they would become non-people.''

Isaacs has heard all the arguments about why imprisoning men in their hundreds on small Pacific islands and leaving them in limbo is the only way to ''solve'' the asylum seeker problem.

But nothing convinced him the cruelty inflicted in the process was worth it. There were moments of heartbreak for the young, untrained Australian, facing the anguish of these men.

Reza, an internee to whom he'd been giving private English lessons, nearly succeeded in taking his own life with a toxic cocktail of cleaning fluids, mosquito repellent, antidepressants and sleeping tablets.

One of the camp's most respected religious leaders, Ali, descended into three days of psychotic madness which left him rolling in the dirt and barking like a dog before he was removed by health workers.

For Ali's Iranian countrymen, it was deeply shocking to see their revered mullah, a man who until then had been a source of succour, reduced to the state of a rabid animal. Later, Isaacs discovered that Ali had just lost a child, having tried, and failed, to transfer funds back home for the sick youngster's treatment.

A third moment of heartbreak came when the camp poet, Pez - whom Isaacs had befriended - tried to hang himself in the laundry.

Isaacs did his best to lighten the long days with recreational activities for the men, but was left feeling as though he was working in a ''death factory''.

''There was no way I could deny I was a part of it, because there I was and it was my country and my people and we were putting these men through such torture'' he said this week.

''It doesn't matter who you are, or what side of politics you are on, if you had been in the position I was in, having to sit there and have a man's friends show you the cord that he tried to hang himself with, crying with them, the rain coming down … it was overwhelming.''

''The camp was built around destroying men,'' he writes, '' grind[ing] them into the dust.''

Isaacs was not a Salvation Army member. He'd done some writing for Oxfam and was working for the state government when a friend told him the Salvos were urgently seeking staff for the hastily reopened Nauru facility. He was hired without even a face-to-face interview.

In passages that will be deeply embarrassing for the Salvos - and for the previous government - Isaacs paints a picture of a camp so hastily set up that the organisations charged with pulling it together virtually had to make it up as they went along.

The new recruits were a ''motley crew'', he says, some of whom were untrained seniors or university students; some only got their contracts as they arrived at the airport to make the trip north. Some were not even sure what an asylum-seeker was.

''I wasn't given any training before I was sent over'' he says. '' No preparation, no cultural diversity training, I didn't know anything about Tamils, or people from Iraq and Iran, and I was one of the comparatively well-informed. There was an 18-year-old there! How can you expect an 18-year-old to look after traumatised, war-torn people?''

In a written response on Friday, the Salvation Army - which has not yet seen the book - said Isaacs was ''engaged … . in a role that required him to fulfil unskilled duties … [and that] support worker roles typically do not require individuals to have particular skills or experience.''

It also said that in the ''early days'' of beginning its work on Nauru and Manus Island, ''the Department of Immigration … required an incredibly rapid start-up, which meant that a formal induction was not developed prior to [the Army] getting its first people on the ground.''

During Isaacs' time there, hundreds more men were poured into the camp - Sri Lankans, Tamils, Iranians, Iraqis, Hazaras from Afghanistan, Pakistanis and Palestinians. The camp ran on myriad rules that were chopped and changed: no hair dye, swimming for inmates banned because of health and safety concerns, no vocational training for internees because that would break the ''no advantage'' rule (i.e. they were not, under the policy, to receive any ''advantage'' over asylum seekers still awaiting processing in UN centres).

Isaacs and his ''recreation'' team tried to defuse the boredom with cricket and soccer competitions.

There were rivalries between some of the staff attached to different aspects of the camp's operations.

''Each agency wanted to be the lead agency, the head honcho of the island,'' Isaacs writes. ''The disorganisation of the Salvation Army meant it was a long way down the pecking order, and the staff suffered because of this.''

Isaacs was mysteriously blacklisted from the camp for a few weeks, and was later told it was because of complaints by security staff that he fraternised too readily with camp inmates.

Staff were repeatedly warned against speaking out about what was going on behind the camp gates. At one stage, Isaacs was told there was an ''intel'' file on him. ''You were not allowed to email your loved ones about what was happening, even what the food was like,'' he said. ''They said that people were checking our emails and Facebook, it felt like being in a horrible fascist state.''

Isaacs acknowledges the Salvation Army got more professional over time at the task it had been rushed into, hiring more qualified staff, and sharply reducing the turnover among contract workers. But he says ''it was the same situation, just wrapped with a prettier bow''.

Isaacs' account is a frank portrayal of the toll on well-meaning Australian men and women who travelled to Nauru to try to ease the inmates' plight. Even coming back for short visits between tours of duty was hard, because few friends really understood the burden staff members were carrying.

Several of the men Isaacs looked after on Nauru have now been moved to the mainland, where they live on bridging visas and minimal benefits. Others have been moved to the Curtin detention centre. Several await court proceedings on Nauru because of their alleged role in last year's riots.

In February this year, the Salvation Army's $74 million government contract to provide welfare services at Manus Island and Nauru was not renewed. ''The Salvos were damned if they did, and damned if they didn't,'' Isaacs says. ''Maybe they are better out of it, because their hands were tied … maybe they could have pushed harder for the men's rights. But then maybe they would not have been able to do the work they were doing.''

Read an exclusive extract of Mark Isaacs' The Undesirables tomorrow in The Sun-Herald.