Already, Mr Turnbull must replace junior ministers Jamie Briggs and Mal Brough who respectively resigned and stood aside over the summer.

The future of Human Services Minister Stuart Robert is also uncertain due to a trip he made to China in 2014 as a private citizen.

An investigation into whether his trip breached the ministerial code of conduct, which is being undertaken by the secretary of the Department of Prime Minister and Cabinet, Martin Parkinson, is also expected on Thursday.

Sources cautioned against writing off Mr Robert. One cautioned that Mr Parkinson's report may not be conclusive, putting the onus on Mr Turnbull to make a call based on "perception".

If Mr Robert is also sacked, Mr Turnbull will have to promote two people to cabinet and four to the junior ministry. Alternatively he could promote three to an expanded cabinet and three to a smaller junior ministry.

The Nationals have been promised an extra cabinet spot, meaning whoever wins the deputy leadership plus one other, will go into cabinet.

Mr Turnbull is being urged to use other promotions to elevate other up-and-comers who also voted for Tony Abbott in order to defuse internal tensions. Victorian Liberal MP Dan Tehan and NSW Liberal Angus Taylor are considered frontrunners for promotion.

Promoting Mr Taylor would also send a signal to the NSW moderate Liberals planning to challenge his preselection to back away. Another member of the NSW right facing a challenge, Senator Concetta Fierravanti-Wells, could also be promoted.


While Mr Truss's retirement has been long anticipated, the departure of Mr Robb, revealed by afr.com, has come as a shock.

As Trade and Investment Minister since the 2013 election, he has been one of the Coalition's best performers, sealing free trade deals with China, Japan and South Korea and most recently the Trans Pacific Partnership of 12 countries.

Mr Robb, who is 64 and entered Parliament with Mr Turnbull in 2004, said he has had a good political career but felt he had one more career left in him. He was eager to go back into the private sector and said "I'd rather do that at 65 than at 68".

"I've never been in better shape, physically or mentally," said Mr Robb who faced a public battle with depression during his political career.

He said the government was in good shape and had a lot of talent coming through and that made it an opportune time to move on.