Simon Bridges has claimed Botany MP Jami-Lee Ross was behind the leak of his expenses report in August.
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But Mr Ross has disputed this in a series of posts on Twitter, and accused Mr Bridges of "unlawful activity".
"The evidence establishes Jami-Lee Ross is the person who leaked the expenses and the sender of the text message," Mr Bridges said on Monday afternoon.
Mr Bridges said he visited Mr Ross on Monday morning with deputy leader Paula Bennett to discuss the report, but was not satisfied with the Botany MP's explanation of events. He said the National caucus will consider Mr Ross' future, including possible suspension.
Mr Ross gave his version of events via Twitter, shortly before Mr Bridges spoke.
"Simon Bridges is going to attempt to pin his leak inquiry on me. He can not 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 (not Tova [O'Brien, Newshub political editor]), as evidence that I am somehow involved.
"I have said they are unrelated - he does not wish to believe that. 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 unlikable in the public’s eyes.
"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."
Mr Bridges said Mr Ross "would say those things" and rejected the claims, particularly of unlawful activity.
"I'm not surprised he's lashing out... he is wrong. He is lashing out. Today I put out the report - it is there and it speaks for itself."
Mr Ross recently went on medical leave. Mr Bridges said this had nothing to do with the leak inquiry.
"I did not know what the investigation report would contain when those matters were addressed in recent weeks," said Mr Bridges.
"Today I have taken steps to ensure that Jami-Lee has the necessary support around him at this time."
The inquiry is being paid for by the taxpayer out of Mr Bridges' leader's budget. It was independently assessed by John Billington QC, and established Mr Ross was behind the leak "on the balance of probabilities".
Mr Bridges says he's not sure how much the inquiry cost.
The details of the leak were embarrassing for Mr Bridges, showing he spent far more money on travel and accommodation than MPs usually manage to chew through - more than $100,000 in three months.
Leaks not plugged
More leaks came on Monday, showing Mr Bridges filed two donor returns in January - the second one lacking two donations which the party says were mistakenly put on his return instead of that belonging to the party.
The donations came from a company with close Exclusive Brethren links and a group called Cathedral Club. Both amounts - $14,000 and $10,000 - are above the threshold which requires them to be made public if donated to an MP, but can be kept secret if made to a party.
The first return was filed on-time in mid-January. The second, amended return was filed in February, after the deadline. National says it refiled the returns on the advice of the Electoral Commission.
The Electoral Commission said in a statement there is no penalty for amending and refiling a return after the deadline.
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The source of the latest leak is said to be a National Party insider who wants him gone as leader.
"I feel very confident in my leadership. What we have here are the actions of a single Member of Parliament," Mr Bridges said.
He declined to answer questions about what Cathedral Club is.