National's Senior Whip Barbara Kuriger says the party will choose a new leader in the next 24 to 48 hours.

Todd Muller has resigned as National Party leader after less than two months in the role.

In a statement, Muller said he had taken time over the weekend to reflect on his experience over the last several weeks as leader of the opposition.

"The role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective," he said in a statement.

Nikki Kaye is now the acting leader of the National Party.

She isn't ruling out putting her hand up to takeover the role, following Todd Muller's shock exit.

Kaye has arrived at Parliament, where an emergency caucus meeting is being held this evening.

She says the most important thing right now is Mr Muller's welfare.

Kaye says the country needs a National government, and the caucus will work through the next steps tonight.

Meanwhile, former leader Simon Bridges wouldn't rule out putting his hand up to take over the leadership.

When asked whether he would like the job back he said "let's see what happens".

Speaking to RNZ at Auckland Airport, Judith Collins wouldn't say whether she's ruling herself in or out of the running for the top job.

National MP Mark Mitchell hasn't ruled out putting his hand up to take over as leader of the party either.

Mitchell threw his hat in the ring for the leadership when Bill English stood down in 2018, but Simon Bridges was chosen by the caucus instead.

He won't say if he will put himself forward again.

Mitchell said the party doesn't want to be in this position, but he has confidence National can win September's election.

Outgoing National MP Sarah Dowie, of Invercargill, says the party's leadership coup in May was a mistake and Simon Bridges should've remained as leader.

Dowie says the situation is unfortunate and she is sad for Muller.

She wouldn't say who she will back to take over as leader.

National MP Louise Upston told reporters at Wellington Airport Todd Muller's resignation was a "shock".

She said the party wouldn't rush to select a new leader.

"If we make a decision today that's great, if we don't it's really important for the country to know the National Party has their best interests at heart, so we will be selecting a leader that we believe has the attributes and ability to lead our fantastic party for now and in the future," Upston said.

Upston wouldn't comment on who she would like to see lead the National Party.

Speaking to RNZ at Auckland Airport, Judith Collins wouldn't say whether she's ruling herself in or out of the running for the top job.

She says she's leaving the decision up to the caucus and she feels "really sorry" for Todd Muller.

Collins, who has been an MP for 18 years, told RNZ in June that she did not have any personal leadership aspirations instead being happy as a valued member of a team.

List MP Simon O'Connor ruled out running for the leadership himself and would not indicate who he would back.

"[Todd Muller's resignation] came out of the blue but I'm looking forward to some very robust discussions today to take the party forward.

"I think you can take one thing for certain that I won't be running for the leadership."

Asked what he thought of Muller's resignation Paul Goldsmith said: "very disappointed".

Goldsmith said the plan now is to "get back on track" as quickly as possible.

Gerry Brownlee would not speculate on who he would support but said he respects Todd Muller's reasons for stepping down.

"Mr Muller is a very fine New Zealander, a man I've got a great deal of respect for. This is not a very pleasant situation that we find ourselves in," Brownlee said.

"My heart goes out to him and his family and I hope that everyone respects their time together.

"There'll be plenty of time in the future to reflect on all of this."

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