Judith Collins announced as new National Party leader, Gerry Brownlee new deputy leader

Senior National MP Judith Collins has been announced as the new National Party leader following a shock resignation from Todd Muller.

Collins, 61, will take on Prime Minister Jacinda Ardern at the September election and senior National MP Gerry Brownlee, 64, will be her deputy leader.

Collins has been an MP since 2002 and currently holds the Papakura electorate. She was a government minister under former National Party Prime Ministers John Key and Bill English, and was a lawyer prior to entering politics.

Born in Hamilton, Collins was one of six children. She was a Labour Party supporter from childhood, but by 2002 had been a National Party member for three years.

"I feel privileged to have been chosen to lead the National Party at this important time in our history," Collins said in a statement on Tuesday night. 

"My focus as leader will be helping rebuild our communities and dealing with the economic and jobs crisis by getting Kiwis back to work.

"Only National has the experience and skills to get us through this. We are a strong team and I look forward to forming the next Government."

Collins has taken the leadership role from Muller after he suddenly resigned from the National Party helm on Tuesday morning, saying in a press statement it had "become clear" to him that he was "not the best person" for the role.

"It is more important than ever that the New Zealand National Party has a leader who is comfortable in the role," he said in the statement at 7:30am.

"The role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective. For that reason I will be stepping down as Leader effective immediately."

Muller said he intended to take some time out of the spotlight to spend time with his family and restore his energy before connecting with his community.

Collins has run for the National Party leadership before.

She announced her bid for leader of the National Party in February 2018, along with Simon Bridges, Amy Adams, Mark Mitchell and Jonathan Coleman.

But Collins lost to Bridges who led the National Party until May this year when he and his deputy leader Paula Bennett were rolled by Muller and his deputy Nikki Kaye.

Collins did not rule out running for the leadership role when Newshub approached her at the airport on her way to Wellington for the emergency National Party caucus meeting on Tuesday.

In her first speech as National Party leader at Parliament in Wellington, Collins explained why she wants to beat Ardern at the upcoming election.

"We are a team that is absolutely determined to take the fight to the Government and to represent the people of New Zealand in a way that they're not represented by the current Government," Collins said.

"We are going through a period of quite difficult times from an economic point of view, as a country, and it is absolutely imperative that the New Zealand people have a good policy platform and a choice as to who is going to lead them through what is starting to look like what is one of the worst economic times in living memory."

Collins said she was "very, very" pleased to have the "honour" of being elected leader.

She said she will not let the Prime Minister get away with any "nonsense".

"I think it's important we give credit where it's due. But at the same time, there is no chance at all I'm going to let Prime Minister Jacinda Ardern get away with any nonsense when it comes to our economy and doing her job," Collins said.

"I will hold her to account."

Brownlee said his job is to support Collins and the rest of the team. He ruled out the prospect of becoming leader.

"It's an opportunity that has arisen and I've put myself up to the caucus and have been selected."

Collins said Paul Goldsmith will stay on as National's finance spokesperson. She also said Muller is welcome to stay on as an MP.

Collins said it was unlikely that there will be a major caucus reshuffle because she doesn't want to unnecessarily disrupt her MPs in their portfolios.