Former National Party leader Simon Bridges has tonight been demoted and "relieved of his portfolio responsibilities" following an allegation of serious misconduct relating to an interaction with a female caucus colleague.

National leader Judith Collins announced Bridges' demotion in a release just after 9pm tonight.

The MP who laid the complaint against Simon Bridges is understood to be North Otago MP Jacqui Dean. It is understood to relate to crude comments Bridges made in front of several people, about five years ago.

Dean complained about it at the time and Bridges was spoken to. Bill English was the leader of the party at the time. Dean has been approached for comment.

"This evening, with unanimous support of the board of the National Party, Simon Bridges, Member for Tauranga, has been demoted and relieved of his portfolio responsibilities," Collins said in a statement.

"The decision follows an allegation of serious misconduct relating to Simon Bridges' interaction with a caucus colleague."

It comes as speculation rises that Bridges was preparing to roll Collins as leader of the party.

The written statement from Collins said the incident relates to comments made by Bridges to a female caucus colleague at a function a number of years ago.

"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," Collins said.

"This decision has not been made lightly. However, the seriousness of the situation demands a swift and decisive response.

"Under my leadership, the National Party will not tolerate harassment and intimidation of any person.

"Members of Parliament and staff should be able to conduct their duties at all times without feeling unsafe or intimidated, and all deserve to be treated with absolute respect by their colleagues in all situations."

Other caucus colleagues reacted with surprise, when approached by the Herald. One said it was a "complete stitch-up" of Bridges by Collins.

National MPs were already expected to likely have an awkward caucus meeting today after a poll showing Collins had lost the support of both National voters and the country at large.

A Newshub-Reid Research poll asked whether Kiwi voters preferred Bridges or Collins to lead the National Party.

Bridges was backed by 40.7 per cent of voters, with Collins having the support of just 23.3 per cent - the remainder didn't know.

Bridges, 45, is the MP for Tauranga.

A former Crown Prosecutor, he has held the seat since 2008.

Bridges was also a former Transport Minister and Energy Minister, and the leader of the National Party from 2018 to 2020.

As Bridges' popularity took a hit in 2020, he was ousted as leader by fellow Bay of Plenty MP Todd Muller.

It is not the first time Collins has shown a ruthless streak - in June, Collins called an emergency late-night caucus meeting of MPs and forced former leader Todd Muller to announce he would resign from politics after she learned he had bad-mouthed MP Harete Hipango to a media company.

At the time, Collins said she would not tolerate MPs' ill-discipline and bad-mouthing each other.

The Herald understands there will be a press conference tomorrow at 10am with Collins, Deputy leader Shane Reti and the complainant. The complainant is still an MP.

Bridges today posted on his Facebook page thanking people for visiting him at Whitcoulls Lambton Quay and buying his book. He was pictured signing books.

It is not the first time Collins has shown a ruthless streak in dealing with her National colleagues.

In June, the National leader called an emergency late night caucus meeting of MPs and forced former leader Todd Muller to announce he would resign from politics after she learning he had bad mouthed MP Harete Hipango to media outlet Newsroom.

At the time, Collins said she would not tolerate MPs' ill discipline and bad mouthing each other.