Two Australian cabinet ministers have resigned over unrelated scandals.

Minister for Infrastructure and Regional Development Jamie Briggs quit after a public servant complained about his behaviour at a Hong Kong bar.

Special Minister of State Mal Brough quit pending a police inquiry into his role in another politician's downfall.

With two front bench cabinet positions to fill in a day, this is the first major setback to Prime Minister Malcolm Turnbull since he took office.

Mr Briggs said he went to a crowded bar after dinner while on a trip to Hong Kong in late November with his chief of staff, some officials and a female public servant.

"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 did in fact occur," he said in a statement.

"However, in the days following the evening the public servant raised a concern 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."

Mr Briggs said at a press conference that he had spoken to Mr Turnbull, and that the prime minister felt Mr Briggs had not met the standard of behaviour required of ministers.

He refused to comment on the specific nature of the incident and did not name the public servant who made the allegations.

Minister for State Mal Brough has been under pressure for months over allegations of illegal diary copying

Mr Brough, who is also Minister for Defence Materiel and Science, is being investigated by police over the alleged illegal procurement of another politician's diary.

He is accused of illegally obtaining copies of former speaker Peter Slipper's diary in 2012 at a time when Mr Slipper was embroiled in a sexual harassment case, which was later dropped. Mr Brough denies wrongdoing.

Prime Minister Turnbull issued a statement saying that Mr Brough had "done the right thing" in stepping aside while police investigated the allegations.

In the same statement, Mr Turnbull said he was "disappointed" with Mr Briggs' conduct but looked forward to his future contributions to the government.

Environment minister Greg Hunt will take on Mr Briggs' infrastructure and regional development portfolio. Minister for Finance Mathias Cormann will act as Special Minister of State, while Minister of Defence Marise Payne will act as Minister for Defence Materiel and Science.

(BBC)