Mr Muller, 51, was installed as the leader of the National party in May after a coup to topple unpopular leader Simon Bridges but lasted just 53 days, shocking parliament by resigning in July.

At the time he walked away without giving comment beyond a statement that "the role has taken a heavy toll on me personally, and on my family, and this has become untenable from a health perspective".

In a lengthy and heartfelt Facebook post on Thursday, timed for Mental Health Week, the Bay of Plenty MP said he suffered "a panic attack for the first time in my life" just a week into the job.

"It started with an intense prickling sensation in my head, followed by what I would describe as 'waves' of anxiety," the former dairy executive wrote.

"I managed to hold it together until I walked into my wife's arms and broke down in a very deep and painful way."

The panic attack came after a string of high-profile blunders, including a decision not to name any Maori MPs in his team's top 12, and his display of a Make America Great Again cap, popularised by Donald Trump, in his office.

Mr Muller said he suffered from sleep deprivation and further anxiety attacks, which sleeping pills and anti-anxiety medication could not ease.

"My family bore the brunt of it, for when I returned for a day on the weekends, I would have nothing left to give them. I found myself unable to get out of bed and mentally distant," he said.

After resigning on July 14, he was replaced by Judith Collins as leader, but decided to stay on as MP - with Ms Collins naming him as trade spokesman.

"I am now a few months on, and with the love of family, friends, and support from a specialist I am recovering well," he said.

"The pressure in my head has abated, although it will take time to fully heal.

"In terms of advice the only piece I will ever proffer is to share the pain ... I can't emphasise this enough.

"Speak up, it does not so matter to whom, but it does so matter that you do."

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Australian Associated Press