The NSW Greens have referred the state's police commissioner to the police watchdog after it was revealed Scott Morrison phoned him over an investigation into one of his government ministers.
Energy Minister Angus Taylor is under investigation after his office used a doctored document to attack Sydney Lord Mayor Clover Moore's travel expenses.
The prime minister told parliament he rang the NSW police commissioner earlier this week to discuss the inquiry.
On Thursday, Greens MP David Shoebridge said he was referring Police Commissioner Mick Fuller to the Law Enforcement Conduct Commission over Mr Morrison's phone call to discuss the inquiry.
"No one, not the prime minister and not the police commissioner, is above the law and people in high office are expected to abide by high standards," Mr Shoebridge said in a statement.
"When two powerful men have a private chat about an ongoing police investigation then of course questions will be asked and they need to be answered."
Mr Shoebridge said he questioned whether the commissioner should retain the confidence of the premier of NSW.
In a letter to the LECC, Mr Shoebridge said Mr Fuller should have known the prime minister and Mr Taylor were "close political colleagues".
"Given this, it was highly inappropriate for Commissioner Fuller to be disclosing any details of the police investigation into Mr Taylor to Prime Minister Morrison," he wrote.
"It goes without saying that Prime Minister Morrison has a vested political interest in the police investigation failing to find any misconduct by his minister."
Mr Shoebridge said the prime minister's personal relationship with the police commissioner - the pair having been neighbours in the past - is a clear conflict of interest.
"This relationship includes, according to Commissioner Fuller, having been neighbours during which time Morrison would collect his rubbish bins for him," Mr Shoebridge wrote.
"Given this relationship, Commissioner Fuller should have declined to provide any information ... and ensured that he had no direct involvement in the criminal investigation into the prime minister's political colleague."
A spokesman for NSW Police said there was no statement available on the issue.
Australian Associated Press