The National Party is holding a press conference after a backlash against Judith Collins' sudden demotion of Simon Bridges has seen her step down as leader.

Collins was voted out as National's leader after the party was locked in a caucus for more than three hours.

Deputy Dr Shane Reti will be the interim leader of the National Party.

Collins confirmed the news on her Twitter feed.

Collins demoted Bridges to the back benches over a five-year-old complaint about lewd comments at a party function.

Bridges hit back this morning saying it was "truly desperate stuff" from Judith Collins who would go to any length to hold on to her leadership.

MP Simon O'Connor, who is Bridges' brother-in-law, said there must be an immediate change of leadership, saying: "The way that this has been handled is just outright appalling".

National MP Barbara Kuriger said she had lots of questions, and the state of the party was pretty disappointing.