Shell-shocked National MPs are making their way to Wellington for an emergency caucus meeting tonight to begin the process of replacing Todd Muller as leader.

Muller, the Bay of Plenty MP, had been in the job for just 53 days before calling it quits at 7.30am today.

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

National's senior whip Barbara Kuriger said she did not know if Muller would be in Wellington for tonight's emergency caucus meeting.

She told media outside Parliament that she was giving Muller his space.

Kuriger confirmed that National MPs were advised of Muller's decision to step down at 7.30am this morning.

A media statement was issued just moments after the call had commenced, Kuriger said.

Muller's deputy Nikki Kaye - who is now acting leader - said "there was a process to work through" tonight.

"The important thing is we have compassion for Todd at this time while we work through what has been a very difficult time."

Kaye wouldn't be drawn on whether she would put herself forward for leader or who she'd support.

"What's important is the best interest of the country and in my view we need a National government and we're focused on how we achieve that."

Judith Collins has also arrived in Wellington this afternoon and told reporters she was sorry for Todd Muller who had been a "delight to deal with".

When asked if she was going to throw her hat in the ring to become leader, she said: "I'm going to keep all of my discussions within caucus".

When pressed if she wanted to be leader, she again wouldn't say.

"I'm going to keep all of my discussions within caucus," she said again after being asked how many MPs were backing her.

Collins said Muller was "extremely well mannered, respectful and good to work with".

She did not know what went wrong for Muller, but it was a "very sad time" for him and his family.

Asked if she had been waiting a long time for her shot as leader, Collins said today was "just another day".

Her comments were near-identical to remarks she made earlier today.

Fellow National MP Michael Woodhouse wouldn't be drawn on whether New Zealand could see a return of Simon Bridges as leader of the National Party.

He said he hadn't spoken to Muller - but when asked if he'd spoken to Bridges he said "I've spoken to a number of colleagues but I just want to let the process go".

Woodhouse said he was "very concerned" for Muller and his family.

He said he "wasn't going to speculate" on whether the fact he also received private Covid patient data from former party president Michelle Boag had played a part in Muller's resignation.

"Let's just get through these couple of days first," Woodhouse told reporters.

Bridges coy on leadership

Former leader Simon Bridges has consoled Muller and described the job as a "very tough role".

"My thoughts are with Todd Muller & his family. Opposition Leader is a very tough role & I wish Todd and his family the best for the future," Bridges wrote on Twitter.

The tweet was silent on whether he would contest the now vacant leadership.

Speaking to media at Wellington airport, Bridges said Muller's announcement was "really sad".

"It's a tough job being Leader of the Opposition."

Bridges said his thoughts were with Muller and his family.

He said he was also thinking about: "What a tough time it is for the National Party."

He said there were some difficult discussions to be had today.

Asked if he would put his name forward for the leadership, he ignored the question.

When pressed on this question, Bridges said: "These are discussions we need to have ... at a difficult time."

Collins won't rule out standing

Earlier today in Auckland Judith Collins said she feeling really sorry for Muller, describing him as a delight to work with".

She would not rule out contesting the National Party leadership.

"I am just going to have the discussion in caucus," she told reporters.

Asked if this was her moment, she said: "It is my moment to focus on my caucus and the team that I am part of, which is the National Party team".

National MP Tim van de Molen told media outside Parliament that Muller's resignation was "a real shame".

"I feel for the guy and in that respect, I wish him all the best."

It was a difficult time for National but MPs were arriving at Parliament to "sort that out today".

The focus now, according to de Molen, was "getting a new leadership team".

Asked if that meant Nikki Kaye's position as deputy leader was in question, he said that would be discussed at tonight's caucus meeting.

He would not say who he would like to be leader.

He said this process had been "untidy".

"With the change in leadership this close to the election, that's where we need to refocus now and our priority now is on getting that clear message with a new leadership."

Jacinda Ardern replaced Andrew Little as leader of the Labour Party a mere 53 days before the 2017 election and led the party to victory.

National Party president: 'Challenging time'

National Party president Peter Goodfellow said:

"There is no doubt that this is a challenging time for the National Party, with the resignation of Todd Muller earlier this morning.

"Being leader of the opposition is one of the hardest jobs in politics and Todd undertook the role with dignity, humility, and respect.

"On behalf of the National Party board of directors I want to thank Todd for his service as leader of our party. The health and well-being of our people is paramount in everything we do, and we wish Todd, his wife Michelle, and their children well.

"We will be working to elect a new leader as quickly as possible, to steer our Party through the upcoming election."

MP suggests new leader could be selected tonight

Speaking to media at Wellington airport, National MP Matt Doocey said that Muller was a "good mate" of his and he "backed him 100 per cent".

"I'm hugely disappointed for Todd - he's a good bugger."

Doocey would not answer questions as to who he was backing as the new leader.

Asked who was the best person for the job, Doocey said: "You will find that out tonight".

There has so far been no confirmation that a vote will occur tonight, only that National MPs will be meeting.

But he said National would be "staying focused".

Arriving earlier in Wellington veteran MP Nick Smith said that today was a "difficult day for the National Party, but we will get through these difficult times".

New Plymouth MP Jonathan Young said today would be a "very important time for us to consider things" and that National was a "good team" but it had been a "pretty tough week".

It now falls to National's MPs to again select a leader who will lead the bruised party though to election day.

There has already been an emergency call this morning as National MPs attempt to piece together the unfolding events.

But the caucus will meet in person tonight between 7pm and 7.30pm – that meeting will be chaired by deputy leader and now acting leader Nikki Kaye.

A spokesperson for National said they don't know whether or not there will be a leadership vote tonight.

After the meeting, National will release a statement and there will be a media stand-up in Parliament where more details of the meeting will be shared.

But at this stage, there is only speculation as to who might take over.

The likely contenders – Judith Collins, Mark Mitchell and Simon Bridges – have all gone to ground, not responding to the Herald's request for comment.

There had been radio silence from all National MPs for most of the morning.

That is, apart from senior whip Barbara Kuriger who confirmed the caucus would meet tonight to "discuss the way forward".

There will be no more official comment from the party until after the meeting, Kuriger said, but MPs will be door stopped for comment on their way into Parliament and at airports all day.

But MPs are now beginning to land in Wellington airport.

Young said he had "no idea" when the new leader will be announced.

He would not be drawn on who he would be supporting as the next leader but said Muller had worked "very hard, and was a great New Zealander".

"Sometimes these things happen," he said, before adding that leader of the Opposition was a "tough job".

Labour: Muller a man of 'great integrity'

Government minister Chris Hipkins described Muller as a man of "great integrity" and an "honest and decent guy".

Today's resignation showed that politics "can take a demanding toll on you", Hipkins said.

Earlier, Prime Minister Jacinda Ardern expressed her sympathy to Muller through a statement.

"No matter what side of Parliament you're sitting, politics is a difficult place."

Deputy Prime Minister Winston Peters also released a statement in which he "acknowledged the heavy price of trying to lead the National Party today".

He said Muller was a "good man" but then took aim at National.

"Leading a divided and incompetent caucus would have tested even the best leader," he said.

"The National caucus now has the unenviable job of selecting its fourth leader since the Coalition Government took office."

As for Muller himself, it is understood that he was not able to make the emergency teleconference caucus call this morning to tender his resignation.

It remains unknown if he will be flying into the capital for tonight's vote.

This comes after Muller, and the National party, has come under intense scrutiny after disgraced National MP Hamish Walker leaked confidential Covid-19 patient information to media.

Although he said he did it to expose weaknesses within the Government's data management processes, the Herald revealed Walker did it in an attempt to prove comments he made in a rogue press release were not racist.

Muller stripped Walker of his portfolios before writing to the party's board to ask it to strip the Clutha-Southland candidate of his membership.

After former party president Michelle Boag revealed herself as the person who leaked the information to Walker, the first term MP announced his resignation.

But Muller still came under intense pressure to front up over who knew what and when.

The party's health spokesman Michael Woodhouse revealed he had also received the information from Boag, but did not use it.

After a week of being on the back foot and facing intense pressure, Muller began to step away from media appearances.

A spokesperson told Newstalk ZB that Muller was not available for an interview at the end of last week because he needed a "cup of tea and a lie-down".

Days later, he refused to appear on Q&A and instead sent his deputy Nikki Kaye.
And Muller also did not attend National's northern regional conference over the weekend.