National MP Jami-Lee Ross says he has a recording of party leader Simon Bridges discussing with him unlawful activity he was allegedly involved in.
Ross, who has been named as the person who leaked details of Bridges' expenses to the media in August, said his decision to speak publicly had nothing to do with the leak.
Ross also said Bridges' recent announcement that he would be taking medical leave for a few months was an attempt to gag him.
"Some months ago I fell out with Simon. I have internally been questioning leadership decisions he was making, and his personal poll ratings which show he is becoming more and more unlikeable in the public's eyes," Ross said in a statement.
"When I started to become expendable, I confronted him with evidence that I had recorded him discussing with me unlawful activity that he was involved in. Working on his instruction, he asked me to do things with election donations that broke the law.
"This evidence led Simon to push me out on medical leave a few weeks ago. It was essentially an attempt to stop me from speaking publicly.
"I now have a duty to speak publicly on these matters. I will do so in coming days."
Ross, who issued his statement shortly before Bridges held a press conference in Auckland to announce the outcome of an inquiry into the leak, said Bridges was attempting to "pin his leak inquiry on me".
"He cannot find who the actual leak is, so is attempting to use contact with my local Police area commander, and a journalist that is a friend as evidence that I am somehow involved.
"I have said they are unrelated - he does not wish to believe that."
Bridges said he was not aware of any recording and rejected any allegation of unlawful activity.
"It's simply not true."
Later, a spokesman for Bridges said: "Fundraising is a normal part of the political process and Simon has had numerous discussions with his front bench colleagues on the issue. Jami-Lee's claims are a matter for him but Simon rejects them. The other matters will be discussed at caucus tomorrow."
Bridges said earlier today it was a clerical error that two donations with a combined total $24,000 of donations were recorded as donations for him instead of the party.
Bridges received two donations, one of $10,000 from Cathedral Club and another of $14,000 from Cubro Ltd, earlier this year.
A National Party spokesman said the donations were for the party, not Bridges.
"In the process of preparing both the local candidate and party returns, as required by the Electoral Act, two donations were incorrectly attributed to Mr Bridges' candidate return by the local Tauranga Electorate Committee."
The error was discovered soon after lodging the candidate return, and the Electoral Commission was contacted immediately to seek advice, the spokesman said.
"Acting on that advice Mr Bridges' candidate return was amended, and then re-submitted to the Electoral Commission for publication.
"The National Party's complete donation return is independently and comprehensively audited."
He said the two donations were "completely unrelated to each other".
Speaking to Newshub this morning, Bridges downplayed the error.
"This is very straightforward – there is party and there is candidate, these [returns] were wrongly filed as candidate, they shouldn't have been and then they were refilled as party, it's as simple as that."
He said it was a "local person who was in charge of these things" who accidentally filed the donations into the wrong place.
"There has been nothing inappropriate there. It was a mistake," he said, adding that it was corrected "very quickly" after National received advice from the Electoral Commission.
Bridges wouldn't go into detail about what the Cathedral Club was but did say Cubro was a healthcare business in Tauranga.
Public disclosure thresholds for candidate donations and party donations are different - candidates have to disclose all donations of more than $1500 while parties disclose those of more than $15,000.