Stunned National MPs will meet tonight to discuss replacing Todd Muller as leader after his shock resignation today.

Tonight's caucus meeting was arranged to "discuss the way forward", senior whip Barbara Kuriger said.

"Our thoughts are very much with Todd and his family at this difficult time as is our compassion and love for Todd," Kuriger said.

"No further comment will be made until after the meeting."

Veteran Nelson MP Nick Smith said today was a "very difficult day" for the National Party "but we will get through these difficult times'".

To Muller, he said: "Stay well, my friend."

Muller quit today as leader for health reasons, plunging the National Party into turmoil just 67 days before the general election. He plans to remain an MP.

"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.

National MPs were understood to be shocked by this morning's announcement.

Prime Minister Jacinda Ardern said in a statement that she just heard about Muller's resignation: "No matter what side of Parliament you're sitting, politics is a difficult place. I have passed on my best wishes to Mr Muller and his family," she said.

Muller: I'm not the best person to lead National

Muller revealed in a statement this morning that he was stepping down as leader "effective immediately". He said he was "not the best person" to be leader.

Muller was leader was for just 53 days after rolling Simon Bridges.

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Early contenders to replace him are his current deputy Nikki Kaye - who is now the acting leader - and the prospect of Judith Collins also lurks in the background.

"It has become clear to me that I am not the best person to be leader of the Opposition and leader of the New Zealand National Party at this critical time for New Zealand," Muller said today.

"It is more important than ever that the New Zealand National Party has a leader who is comfortable in the role."

Muller says the role had taken a "heavy toll" on him personally.

He intended to take some time out of the spotlight to spend with his family and to restore energy before reconnecting with his community.

Newstalk ZB political editor Barry Soper told Mike Yardley that Muller's resignation was "absolutely astounding", even though Muller was having a hard time in the fallout of the Hamish Walker issue.

"Muller has realised how hard leadership is when the focus of the media's on you. He believes the National Party should be led by someone who is more comfortable in the role.

"It is a bombshell so close to the election. It's extraordinarily unfortunate for the National Party."

Soper believes the National Party will look to Judith Collins as leader, but says it will be somewhat of a poisoned chalice for her.

On July 8, Muller's press secretary declined an interview with Newstalk ZB Drive host Heather du Plessis-Allan because he was "having a cup of tea and a lie-down."

The latest UMR poll, leaked to the Herald, had National on 32 per cent, down 2 percentage points and well behind Labour on 53 per cent support.

National was on 38 per cent support in the latest 1 News-Colmar Brunton poll on June 25, but the party's own internal polling at the end of June had National stagnating at about 34 per cent for the past three weeks.

Muller had been at 13 per cent in the preferred PM stakes in the 1 News-Colmar Brunton poll.

Dunne: Judith Collins best option for National

Former MP Peter Dunne told Newstalk ZB that Collins was in a strong position to take the leadership, even though this year's election was likely to be lost.

"It's staunching the wound that's important... she's probably best-placed to do that."

Backbench MPs would today "be looking to a leader who can give them the best protection" at the election, he said.

Dunne believed one of the biggest beneficiaries could be the Act Party as voters looked for right-of-centre alternatives, given National's troubles.

On Muller, Dunne said he had "clearly good intentions". He was "a good honest person, but the job proved to be far too much for him".

"There was frustration with the situation beforehand [with Simon Bridges] so any change had to be a good one even if it wasn't well thought through."

Right-wing political commentator Ben Thomas told Newstalk ZB that Muller's resignation was a surprise.

"There was nothing resignation-worthy of the events of last week."

Thomas speculated there might be a "smoking gun" coming.

"He was clearly struggling with caucus discipline," said Thomas, referring to the Hamish Walker scandal.

Thomas believed National now needed someone straight out of the box – including possibly Simon Bridges.

"I think he will fight for his job back."

The other leading contender was Collins, Thomas said.

Winston Peters: Muller a 'good man'

NZ First leader Winston Peters said he had sympathy for Muller.

"Todd is a good man, unlike most of his colleagues he does have commercial experience, and he will bounce back," Peters, who is also Deputy PM, said in a statement.

"Leading a divided and incompetent caucus would have tested even the best leader. The National caucus now has the unenviable job of selecting its fourth leader since the Coalition Government took office."

Peters then put the boot into the rest of National's caucus.

"The National caucus, like too many parties in Parliament, lacks business experience, life experience and political experience. Heaven only knows who will be the next cab off the rank selected to lead such a dispirited and incompetent lot.

"Todd never had a chance given the fault lines of ambition, personality, and ideology that run deep through the National Party caucus. National has demonstrated to voters as clearly as it is able that it cannot govern itself."

Peters said National's "instability and hubris takes it out of the running" in the upcoming election.

Act leader David Seymour said National had several options for leader but how they chose in the next 48 hours would have a major impact on New Zealand "because we need to have a competitive election".

"You have to ask, at what point do you bring Simon Bridges back. He got to 40 per cent polling."

Seymour said it was important to "recognise the human side of politics".

He said you could see at times Muller got exasperated when someone was putting the boot in. He believed that was because Muller hadn't had experience in a competitive environment with his background in Zespri and Fonterra.

"Todd is a really nice guy but he's grown up in a series of monopolies and being a leader is competitive."

Muller's rough start to role as leader

Muller's integrity had come into question last week when he was repeatedly questioned about whether the party's health spokesman Michael Woodhouse had received any confidential information from former party president Michelle Boag and he replied that Woodhouse hadn't when he knew that he had.

He later said that he thought he was being asked about whether it was the same information that was given to Clutha-Southland MP Hamish Walker and he had been "transparent".

Muller later said he could have been clearer about his answers about what he knew about Woodhouse and Boag.

Boag, who has resigned from the National Party, today declined to comment on Muller quitting.

The scandal had diverted attention away from Muller's speech about National's economic plan, and he was going to attempt to wrestle back control of the narrative with a major speech today about infrastructure.

Muller and Kaye mounted a challenge for the leadership and deputy leadership of the National Party in May.

An emergency caucus meeting on May 22 to determine the party's leadership resulted in Bridges and former deputy leader Paula Bennett losing their positions.

Muller encountered a baptism of fire as National leader, including criticism over having a Donald Trump Make America Great Again hat and fallout over a perceived lack of diversity on his frontbench.

On June 29, Bennett announced she was bowing out of politics and venturing into the "business world". She had also been removed as the party's campaign chair and replaced by Gerry Brownlee.

Muller's front bench selection was criticised for lacking Māori representation. His highest Māori MP was Paula Bennett, who subsequently retired from politics, and number 13, and she was replaced by Shane Reti.

Then the party was consumed by the scandal over the leaking of Covid patients' private detail.

The fallout led to Clutha-Southland Hamish Walker resigning from the party.

National's Anne Tolley also announced on June 27 she would retire after 34 years in politics. At the time, she said she was confident National had the team and the leadership to win the election on September 19.

On July 10, National list MP Dr Jian Yang announced his retirement from politics and said he would not stand in the 2020 general election after three terms in the party caucus.

A week of questions over Muller's handling of the leak scandal

July 2

• Evening: National MP Hamish Walker sends the Herald a list containing names, dates of birth and isolation facilities of then 18 active current Covid-19 cases, to use as evidence he was right in saying cases were coming from India, Pakistan and Korea but does not reveal the source.

July 4

• Weekend Herald reveals there has been a privacy breach in that it has been sent details of current Covid cases without revealing who sent it or why. Two other media outlets reveal they have been sent the same information. None publishes the info.

• Health Minister says there will be an inquiry.

• National's health spokesman Michael Woodhouse recognises the description of the information similar to info sent to him by party stalwart Michelle Boag in four emails between June 21 and 25.

• Woodhouse texts Boag to say he had not sent his info to the media. Boag says she knows because the info is different. The number of active cases Woodhouse was sent in June were lower than the 18 active cases Walker was sent but Boag does not tell him it was Walker.

• National leader Todd Muller and Woodhouse strongly criticise the Government for the privacy breach.

July 6

• Lunchtime: Walker contacts Muller privately to confess to having forwarded Covid-19 patient information to the media.

• 3pm: Govt announces independent inquiry to be led by ex-Solicitor General Michael Heron, QC.

July 7

• Muller spends day in Dunedin with Woodhouse.

• Walker issues statement about 5.30pm admitting he sent the patient details to media but says it was to expose the Government's shortcomings at keeping data.

• Boag issues statement moments later admitting she sent the patient details to Walker. Resigns from Auckland Rescue Helicopter Trust CEO, in which capacity she got the data from the Ministry of Health.

• Muller issues statement saying it was an error of judgment by Walker, that he has lost his spokesmanships, that he must co-operate with the inquiry and that he, Muller, will not be commenting further until the inquiry is over. Resignation is not mentioned.

• Woodhouse tells Muller he, too, has received similar information from Boag in June but did not do anything with it.
• Woodhouse deletes the Boag emails.
July 8
• Muller says he is angry with Walker and has referred the matter to National's board of directors.
• Walker announces retirement from politics before board deselects him.
• Woodhouse contacts Heron to say he was sent similar information to Walker from Boag.
July 9
• Muller has standup after speech and rejects a suggestion multiple times Woodhouse might have received similar information from Boag.
July 10
• Woodhouse publicly says he received patient information from Boag on four occasions between June 21 and 25 but did not do anything with it or tell anyone about it.
• Boag resigns from the National Party.
• Muller denies having made misleading statement the day before regarding Michael Woodhouse. "From my perspective, we have been well-managed and transparent about it."

July 14
• Muller resigns as leader of the National Party.

Muller's full statement:

Todd Muller resigns as Leader of the Opposition

I have taken time over the weekend to reflect on my experience over the last several weeks as Leader of the Opposition.

It has become clear to me that I am not the best person to be Leader of the Opposition and Leader of the New Zealand National Party at this critical time for New Zealand.

It is more important than ever that the New Zealand National Party has a leader who is comfortable in the role.

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.

I intend to take some time out of the spotlight to spend with family and restore my energy before reconnecting with my community.

I look forward to continuing to serve as a loyal member of the National Party team and Member of Parliament for Bay of Plenty.