Speculation was rampant among Liberal MPs on Saturday that Defence Minister David Johnston could go, though well-placed sources suggested he would not be pushed from the job and pointed to Prime Minister Tony Abbott;s previous reluctance to make sweeping changes to his frontbench.

Immigration Minister Scott Morrison refused to buy into speculation he could be handed a new role if Senator Johnston was moved on when contacted by Fairfax Media.

"Serving in the cabinet is a great privilege," he said. "We do so at the request of the Prime Minister in whatever role he asks us to, in service of the Australian people. Our responsibility as ministers is to get the job done. Our performance against that criteria is the only judgment that matters."

Senator Johnston is a political ally of Foreign Minster Julie Bishop – the pair are leading lights of the moderate faction in Western Australia – but some in Coalition ranks have suggested that following a series of gaffes and missteps, Ms Bishop now accepted her political ally may have to go.

Fairfax Media spoke to Liberal Party cabinet ministers, junior ministers and backbench MPs on Saturday, and most suggested Mr Frydenberg was the man most likely to replace Senator Sinodinos.

A former staffer to Alexander Downer elected to Parliament in 2010, he was promoted as the Prime Minister's parliamentary secretary shortly after the Coalition's win in 2013.

Mr Frydenberg has overseen the government's deregulation agenda in repealing red tape.

Senator Sinodinos quit the frontbench on Friday after Fairfax Media revealed on Thursday night that  Mr Abbott would spend the weekend pondering a year-ending reshuffle.

Senator Sinodinos said he had decided to quit because ICAC reports into the Credo and Spicer matters, in which he was called as a witness only, had been delayed from January 2015 until March or even later.

"While I am extremely disappointed by this further delay, I am confident that when the reports are finally released, I will not be subject to any finding of corrupt conduct or illegality," he said, adding that the government needed an Assistant Treasurer in place before its second budget in May.

He also took aim at the leak of his departure and said they "did not assist the orderly workings of government", urged the promotion of more women and said ministerial problems should not be allowed to "fester".

Parliamentary secretary Steve Ciobo, who is close to Treasurer Joe Hockey, is the other possibility for the Assistant Treasurer's job.

One of two women – Mr Morrison's deputy Michaelia Cash, or Assistant Education Minister Sussan Ley – is tipped to move up from the outer ministry into cabinet to take Mr Morrison's old job.

Mr Frydenberg has moved rapidly through the ranks since being elected.

He will be the first member of the class of 2010 to enter the ministry.

Melbourne-born, he has law and economics degrees from Monash University, a master's of international relations from Oxford and a master's of public administration from Harvard.

Politically, he is close to Environment Minister Greg Hunt, Mr Morrison and Families Minister Kevin Andrews.