National MP and former leader Todd Muller announced his retirement at the next election after admitting to his party he had been an anonymous source in an article criticising returning MP Harete Hipango.

Muller says in a statement he needed to prioritise his health and family, and wanted to give the National Party plenty of time to select a new candidate for his seat.

But it has subsequently emerged that Muller's resignation followed a late-night admission to his fellow National Party MPs that he had been one of the anonymous sources in a Newsroom article criticising Hipango, who is returning to Parliament following Nick Smith's resignation.

It's understood that this made National leader Judith Collins furious, and there was a threat that Muller could be expelled from caucus.

The caucus meeting was sparked by a different story concerning allegations of inappropriate spending by Hipango, which Collins has put down to an innocent 'coding error”.

National leader Judith Collins. Image: RNZ/Dom Thomas.

Collins was coy when asked about whether she wanted Muller to resign on Wednesday, saying the decision was not in her hands.

"I think it shouldn't come as much of a surprise to people."

She declined to comment any further when asked about the late-night caucus.

Muller, who was very briefly leader of the National Party after ousting Simon Bridges in a coup, did not respond to requests for comments.

He denied leaking when asked by the NZ Herald but says he had made a 'comment to a journalist that had subsequently been quoted”.

'This has been a difficult decision because being a member of Parliament is a huge privilege, but it does come at a cost,” says Muller.

'I have decided that I need to prioritise my health and family, and move on to the next chapter of my life.”

He says in the statement the highlight of his career is serving the local community and working with Climate Change Minister James Shaw on achieving cross-party support for the Zero Carbon Act.

'I can assure them all I won't stop advocating for mental health facilities, after hours A&E and roads for my remaining time as their MP,” says Muller.

'At a national level I am very proud of the work I did with James Shaw in creating bi-partisan support for climate action and achieving cross-party support for the Zero Carbon Act.”

Muller's Bay of Plenty seat is considered very safe for the National Party, meaning the race to become the National candidate in the seat will be hard-fought.

'In announcing today, I hope to give the National Party and my local branches ample time to find their next candidate,” says Muller.

'The National Party needs to take the opportunity this term, to renew and refresh, with strong candidates for the next election.”

Muller is on leave for the next five weeks to care for his wife, who is undergoing what he described as a 'significant medical procedure”, and will return after the three-week winter recess.

'I look forward to working out my term with the National team. In the meantime, I am on pre-approved leave for the next five weeks, to care for my wife who is undergoing a significant medical procedure. I will return to Parliament after the winter recess.”

Muller was elected in 2014 following a business career at Zespri and Fonterra, and a stint working for former National Party prime minister Jim Bolger.

His name had long been thrown around as a potential leader for the National Party and he eventually ousted leader Bridges after sparking a leadership contest in May of 2020.

Muller was leader for just 53 days however, stepping down in July, citing severe mental health strain.

He says he had a series of panic attacks before stepping down.

'It just came in these utterly debilitating waves. I was sweating, dry-retching – it was just hideous. I'd go to the bathroom and splash some water on my face and do some breathing exercises, because I knew that helped. But it was absolutely excruciating.”

Henry Cooke/Stuff.