Mr Turnbull declared his visit to Washington DC a success after "frank and productive meetings" with President Donald Trump, military leaders, business executives and more than 40 state governors.

Mr Turnbull and his wife Lucy departed the US for Canberra on Sunday morning AEDT.

He'll have a new mate beside him on the frontbench when the Nationals elect a leader to replace Barnaby Joyce, who quit the role and the deputy prime ministership on Friday.

The government is hoping Mr Joyce's decision, after a fortnight of political and media attention over his extra-marital affair with a staffer, will be a circuit breaker.

A Sky News/ReachTEL poll on Sunday had Mr Turnbull's and the government's approval ratings slipping on the back of the Joyce affair.

The new Nationals leader and deputy prime minister is expected to be little-known Veterans Affair Minister Michael McCormack, who'll probably be asked about the coalition deal with the government at his first question time.

The federal opposition is also still interested in whether Mr Joyce breached any ministerial standards, or expenses guidelines, over his affair with Vikki Campion, who is pregnant with their child.

Labor will also exploit the battle between former prime minister Tony Abbott and his former cabinet colleagues. They expect Mr Joyce to follow suit.

The outspoken MP has promised not to "snipe" from the backbench.

"They will be like Statler and Waldorf from the Muppet Show, the government's greatest critics, trying to bring Malcolm Turnbull and the government down," Labor frontbencher Jason Clare said.

The former prime minister has used the parliamentary break to criticise ministers over their support for higher migration, and says the Joyce affair has been poorly managed "at the most senior levels of government".

Meanwhile, Senate estimates hearings will probe ministers and senior officials from government agencies and departments.

Australian Associated Press