Newshub can reveal that Simon Bridges made an urgent demand of the country's most powerful police officer - requesting he reveal the identity of the leaker of his expenses.
But police turned him down, warning that to do so would endanger the safety of the leaker.
Mr Bridges made his first request to the Commissioner of Police on August 24, and was told police would not disclose that information "in the interests of privacy".
But that didn't cut it for the National Party leader.
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Newshub can reveal that three days later, his Chief of Staff wrote a tersely worded letter to the Commissioner, which read:
"It is illogical that NZ Police have the identity of this person, but have not released that information to National."
Police fired back, denying Mr Bridges' request and warning that to reveal the leak would likely "endanger the safety of the person involved in this matter".
The urgent letter was written on the same day Mr Bridges commissioned his witch hunt - the PricewaterhouseCoopers (PwC) inquiry into the leak.
"It was absolutely the right thing to do," he told The AM Show.
But in the letter, his office says the inquiry is a needless waste.
"It is unnecessary and a waste of taxpayers' money to require National and the Parliamentary Service to undertake an investigation when NZ Police know who the person is."
Mr Bridges blamed Jami-Lee Ross for the leak, and the inquiry said the evidence pointed to Ross but couldn't prove it.
Since Mr Ross' unceremonious departure, the National caucus has continued to leak. Mr Bridges says he hasn't changed his mind about who the leaker is.
"I stand by all the positions I've taken over that issue," he told Newshub.
But that hasn't helped to plug the leak.
Mr Bridges' letter to police said he "remains concerned about the wellbeing of the individual involved".
But it also said because the leak was being portrayed as the work of a National MP, it's causing concern amongst the caucus as they feel their reputations are being called into question.