Australia's prime minister rejected calls Monday to suspend a cabinet minister accused of rape, saying the unnamed official "vigorously denied" allegations that have now been referred to police.

Scott Morrison -- already under mounting public pressure to tackle what has been called a toxic sexist culture in Canberra politics -- insisted the minister was innocent until proven guilty. "Allegations of criminal conduct should be dealt with by competent and authorised agencies," Morrison said, adding it was a "matter for the police."

Morrison recently received a letter detailing allegations that a man currently serving in the cabinet raped a 16-year-old girl in Sydney in 1988. The woman took her own life last year. The man not been named publicly due to Australia's defamation laws.

Morrison said he had raised the issue with the minister "who absolutely rejects these allegations." "It is the police, in a country where you're governed by the rule of law, that determine the veracity of any allegations of this nature," Morrison said in a testy exchange with journalists on Monday. "The police have had this matter referred to them. The individual involved here has vigorously rejected these allegations. And so, it's a matter for the police."

Police said they had "received a complaint relating to a historic sexual assault", but "will not be making further comment."

The fact the woman can no longer be questioned and the time elapsed since the alleged attack meant prosecution is unlikely. Opposition senator Sarah Hanson-Young -- who also received the letter and forwarded it to police -- said there was "no prospect" of having the case heard and an independent investigation should be established.

This is just the latest scandal to hit Morrison's government and Canberra politics this year. Defence Minister Linda Reynolds is currently on medical leave amid an investigation into a staffer allegedly being raped by a colleague in her parliamentary office.

The 26-year-old alleged victim Brittany Higgins said she was treated like a "political problem" when she reported the incident, which occurred in 2019 just a month before national elections. Morrison initially defended his government's approach to the case before apologising.

Hanson-Young also revealed over the weekend that she had also referred a rape allegation against an unnamed Labor member of parliament to police.

Australia's parliament has been repeatedly criticised for a "toxic" workplace culture that has allegedly spawned persistent bullying, harassment and sexual misconduct against women.

Last week Nicolle Flint, a prominent female Liberal member of parliament who publicly complained about sexist abuse, announced she would step down at the next election.