Simon Bridges 'highly likely' to challenge Judith Collins for National leadership - party source

Simon Bridges is "highly likely" to challenge Judith Collins for the leadership of the National Party, a source within the party has told The AM Show host Ryan Bridge.

A political firestorm is brewing in the National Party with the shock demotion of MP and former leader Bridges on Wednesday night. Leader Judith Collins night stripped him of his portfolio responsibilities over allegations of serious misconduct.

Collins said it related to an interaction with another colleague. Newshub understands that colleague is Waitaki MP, Jacqui Dean.

"The case relates to comments made by Mr Bridges to a female caucus colleague at a function a number of years ago," Collins said in a statement on Wednesday night. 

"Having been made aware of the seriousness of the complaint for the first time and the ongoing distress this has caused the complainant, I was left with no option but to immediately demote Simon Bridges and relieve him of his portfolio responsibilities."

Political commentator Lara Greaves, from the University of Auckland, said people may be skeptical of the timing, given Bridges earlier this week wouldn't rule out another tilt at the National Party leadership.

But she said it could also be the last straw for Bridges.

"He's been a minister before, he's been [the] leader of the party before - there's not necessarily a lot of reasons for him to stick around," she told Newshub.

"If it's something that is a bit borderline, you can see that he'd be inclined to resign."

Many National MPs were tense and angry as they left Parliament on Wednesday night. Whangaparāoa MP Mark Mitchell told Newshub they were flying blind.

"We'll be meeting tomorrow... I hope that will be happening hopefully first thing tomorrow morning and we'll be able to discuss the events,

"There has not been any caucus meeting yet. It's really important that happens and once we've come together tomorrow, then we'll be able to come out and give you a much clearer picture in terms of what's going on.

"I don't know what's going to unfold tomorrow. There's obviously been a big announcement today, it blindsided most of the caucus who weren't aware of it."

Fellow MP Chris Penk said he wanted to hear from all sides before passing judgement.

"I'm not going to comment on stuff until we've had a chance to hear from all sides and that's really important.

"It's important that we have these discussions in a way that allows everyone to have their say and I'm not convinced that's happened so far."  

Newshub political editor Tova O'Brien said it's believed the complaint related to an incident at a function five years ago.

"National MPs were there including Simon Bridges, Jami-Lee Ross and Todd McClay. They were talking to Waitaki MP Jacqui Dean and the allegation is that something was said to her of a sexual nature that she found both humiliating and designed to dominate.

"She kicked that complaint up to then-leader of the National Party Bill English."

O'Brien said English refused to comment to Newshub on Wednesday evening.

Bridges isn't commenting but his supporters say it's a set up, she said.

Collins will front the media later on Thursday morning.