Prime Minister Malcolm Turnbull's Government has been rocked after two scandal-hit MPs stood down from their frontbench portfolios on Tuesday.

Liberal MP Mal Brough will stand aside, pending a police investigation, while Jamie Briggs has tendered his resignation.

Mr Briggs has resigned as Minister for Cities and the Built Environment following a late-night incident involving a female public servant in a Hong Kong bar during an official overseas visit last month.

The South Australian MP called a press conference on Tuesday to say the public servant took offence at his actions in the bar.

"At the conclusion of the official programme for the day, my chief of staff and I went for dinner and we invited several other officials, of which one female public servant agreed to attend," he said.

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"At the conclusion of the dinner (which I paid for personally) we went to a popular and, as it transpired, very crowded bar for drinks during which we interacted between the three of us and with others in what I believed, at the time, was an informal manner.

"At the conclusion of the evening, the public servant left to return home and my chief of staff and I returned to our hotel together.

"At no point was it my intention to act inappropriately and I'm obliged to note for the record that nothing illegal has been alleged or in fact did occur," he told a press conference.

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"However, in the days following the evening, the public servant ... raised concerns about the appropriateness of my behaviour towards her at the venue.

"I've apologised directly to her but after careful reflection about the concerns she raised and the fact that I was at a bar late at night while on an overseas visit I have concluded this behaviour has not met the particularly high standards for ministers."

Minutes after Mr Briggs's press conference, it was announced Mr Brough would stand aside.

The Special Minister of State and Minister for Defence Materiel and Science has been under investigation by the Australian Federal Police over the alleged copying of the diary of former speaker Peter Slipper.

In a statement, Mr Turnbull said Mr Brough would step down from his ministerial role pending the completion of inquiries by police.

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"In offering to stand aside Mr Brough has done the right thing, recognising the importance of the Government maintaining an unwavering focus on jobs, economic growth and national security," Mr Turnbull said.

In a letter to the Prime Minister, Mr Brough said he made the decision because he has not been given any indication when the AFP's investigation will be concluded.

"It disappoints me that this matter hasn't been resolved by this time as all facts have been in the public domain for years and the public statements of Mr Ashby confirm my position that at no time did I counsel or procure him for any improper purpose," Mr Brough said.

"I look forward to resuming my ministerial responsibilities and assisting with the important work of Government once the investigation is complete."

Mr Brough said the AFP has told him the earliest they could interview him about the allegations was after January 5, 2016.

Minister for Finance Mathias Cormann will act as the Special Minister of State, while Defence Minister Marise Payne will also act as Minister for Defence Materiel and Science.

Minister for the Environment Greg Hunt will replace Mr Briggs as Minister for Cities and the Built Environment.

Mr Briggs, the Liberal Member for Mayo, is expected to remain in Federal Parliament as a backbencher.

"While disappointed by the conduct that led to his resignation, I thank Mr Briggs for his capable service as a Minister," Mr Turnbull said.

"I look forward to Mr Briggs continuing to make a valuable contribution to the work of the Government in the future."

Turnbull 'saved up all bad news' for today: Plibersek

Acting Opposition Leader Tanya Plibersek attacked the timing of the two announcements, claiming the Prime Minister was "taking out the trash".

"He saved up all the bad news to a day between Christmas and New Year when he hopes no-one was listening," Ms Plibersek said.

"First thing we read this morning is the Medicare Services review won't be returning money to the health budget — it's a simple disguise for a cut to Medicare services.

"Then we hear, of course, that the Gonski school funding reforms won't be completed by this Government, [and] now we hear that the PM has lost two of his ministers."

Mr Briggs won a by-election for the South Australian seat of Mayo in 2008 and was the assistant minister for infrastructure and regional development until September.

He recently made headlines for hurting his leg during a party hosted by the former Prime Minister Tony Abbott following the Liberal party leadership spill.

He first claimed the injury was sustained while running, but later admitted he had attempted to crash-tackle Mr Abbott.

The ABC understands there will be no formal reshuffle until the Deputy Prime Minister, Nationals Leader Warren Truss, makes his intentions clear.

There has been speculation Mr Truss will retire at or before the next Federal election.