An MP who competed against Judith Collins for the leadership of the National Party is throwing his support behind his new leader and says it's onward and upward.

Judith Collins has been voted in as National Party leader to replace Todd Muller, who quit unexpectedly yesterday.

National MP Mark Mitchell also put his name in the hat and was grateful for the support he received.

He told Mike Yardley he enjoyed the process and MPs had a robust debate.

Mitchell says the party emerged with Collins as a new leader.

Yesterday was a bruising day for the party, in which Todd Muller quit as leader at 7.30am, MPs made their way to Wellington for an emergency caucus meeting, and Collins emerged victorious as the leader after the contested leadership race against Mark Mitchell.

Mitchell told Newstalk ZB he was "100 per cent" behind Collins. He did not know the voting numbers, but said there was a new voting process. "What happens in caucus stays in caucus."

"Unification now is extremely important and I know Judith understands that clearly and the caucus does as well."

National MPs took two hours late last night to decide Collins should be their new leader, with Gerry Brownlee elected as her deputy. Nikki Kaye, who was Muller's deputy, did not seek re-election for the position.

Listen to the audio above as Mark Mitchell and Stuart Nash speak to Mike Yardley