National MP Jami-Lee Ross is facing suspension from the party caucus after being blamed for leaking Simon Bridges' expenses to Newshub.
The investigation into the leak couldn't definitively identify the leaker, but the evidence strongly pointed to Mr Ross.
Deputy Prime Minister Winston Peters says Monday's events don't look good for Mr Bridges as party leader.
"It's got all the signs, as the Australians say, of a spill going on," he speculated. "As to when it will happen we don't know, but it will be sooner rather than later."
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Mr Bridges has rebuffed that idea, saying he feels "very strong in my leadership".
When his caucus votes on Mr Ross on Tuesday, they'll effectively also be casting a vote on their leader.
Mr Ross is currently on leave to deal with personal health issues, but Mr Bridges and his deputy Paula Bennett visited him on Monday morning.
"I also discussed with Jami-Lee Ross matters concerning his conduct that have come to my attention and suggest, together with the leak, a pattern," Mr Bridges told media.
Mr Ross vehemently denies the allegations, and cut across his leader's big announcement by taking to Twitter five minutes beforehand.
He claimed he was being framed and accused Mr Bridges of breaking the law around election donations. He also said the party leader was "becoming more and more unlikeable in the public's eyes".
"I'm not surprised he's lashing out," Mr Bridges said of the explosive tweets. "He's wrong."
He rejected the accusations of law-breaking as "simply not true".
On Monday morning his leadership was further undermined. A source with links to the National Party told The AM Show that Mr Bridges changed his election donations to swap out two donors.
Mr Ross is the MP for Botany, Auckland. If he's expelled from the party he could quit and cause a by-election, or stay on as an independent.
National MPs will decide his fate on Tuesday.
Mr Ross was once an acolyte of Judith Collins, but didn't support her for the leadership - throwing his weight behind Mr Bridges instead.
Ms Collins says if she was leader, Mr Ross would be gone.
"I wouldn't put up with the behaviour at all," she told Newshub. "My point to Jami-Lee Ross is put up or shut up."
Mr Bridges smiled when asked if he thought Mr Ross was working with any other MPs to try stage a coup.
"I will let you draw your own conclusions on that," was all he would say.
Newshub has now received three leaks from National sources undermining his leadership, starting with the initial expenses in August.