Nikki Kaye knew she was going to leave politics as she walked out of the caucus room on Tuesday night.

The MP for Auckland Central had started the day as deputy leader, was acting leader by that morning after Todd Muller stood down, and had even been pushed by other National MPs to contest the leadership just hours earlier.

But as National MPs emerged from the caucus room behind new leadership team Judith Collins and Gerry Brownlee, Kaye knew she was done.

"I walked out and came along the Parliament forecourt, and I felt this overwhelming sense that now is the time to leave," Kaye told the Herald.

"I just knew. I can't really explain but I knew when I walked out of that caucus room. I knew my chapter was ending."

Speculation is rife that Kaye's sudden and unexpected departure is because she doesn't want to work under Collins or had a role in the leadership team's mishandling of the Covid patient privacy leak.

Kaye dismisses this.

"I'm very confident of the way I've operated and acted throughout this period, with very difficult circumstances arising. It's got nothing in any shape or form to do with that."

And on Collins: "Not at all, in any shape or form. I back Judith Collins 100 per cent to be prime minister. I will campaign for her. She is a formidable politician and I hope she is prime minister."

Nor is Kaye bailing because of the potential for an inquiry to challenge her previous comment that she hasn't received any confidential Covid patient information from close friend and former party president Michelle Boag.

"I've had nothing to do with that issue. I have acted with complete integrity. It has absolutely nothing to do with that.

"People can run all the conspiracy theories they want, but the truth is I'm choosing to take time for me."

Asked to run for leader

It seemed Kaye was at a fork in the road on Tuesday: either contest for the leader's role, or walk away from politics.

As leader, her liberal blue/green credentials might have had the potential to swing the centre behind National.

She is even sometimes jokingly referred to as a Labour MP in the wrong caucus, an idea she scoffs at.

"That is unfair. I'm certainly liberal blue-green, but I'm centre-right.

"The next generation of New Zealanders, many of them are socially-liberal, they care about the environment, but they also want equality of opportunity and the ability to have reward for effort. If anything I've been representative of many of the next generation of New Zealanders."

Did National MPs ask her to challenge for the leadership?

"Some did, absolutely."

She considered it, including the nature of the role and whether she would have caucus support.

In the end, she told Collins that she supported her bid before the vote because it was what was best for the party.

That's been her driving force in the recent months of upheaval for National, she says.

"It's hard for people to sometimes see this, but whether it was potentially Todd standing [with Kaye as deputy], whether it was me making it clear to Judith that she'd be the best leader, I have tried to what is best for the party and the country.

"Judith is the right leader, right now ... To cause disruption [by contesting] when in my view she's the right person would not have been a good thing."

Todd Muller's successful leadership challenge in May saw Nikki Kaye (right) become deputy and the return of Amy Adams (left). Photo / Mark Mitchell

It was not the future she had envisioned when she signed up to be Muller's deputy in May.

"If the circumstances had been different, maybe I would have been Deputy PM, and that would have been the right thing if potentially circumstances hadn't occurred. But they did.

"This is what I've learned about life. You can't know what's around the corner. All you can do is act in the best way possible. That is what I've tried to do."

She doesn't regret being part of the team that rolled Simon Bridges because she didn't think he could win.

But a lot has changed since then, she says.

"The balance of the calculation shifted for me. I had to balance making a significant contribution to New Zealand versus maybe going off and living a little bit.

"I've always chosen my country and my party, but now it's time to choose me."

Part of that calculation, she says, is her unique perspective from having beaten the breast cancer she was diagnosed with in 2016.

"I've tried to think like I've got 40 years but think like I've got 12 months. It's a pretty good way to look at life."

A hospital pass for National

Kaye says she is determined to campaign for National's next Auckland Central candidate and for the party.

"I do accept it is challenging, the timing of my retirement. That is what I've tossed and turned over.

"But it wouldn't have been the right thing to take up the space of someone else potentially in Parliament if my heart is not 100 per cent there.

"I've always said you never ever want a seat-warmer. You've got to have people that are 100 per cent in."

Kaye has been the Auckland Central MP for 12 years, winning what had been a Labour seat for decades, and her ministerial portfolios when National was in power include education, ACC, food safety, and civil defence.

She is proud of second language learning and the roll-out of fast, uncapped internet connections in schools, cuts to ACC levies, and mobile civil defence alerts.

As a local MP, she has progressed a conservation park for Great Barrier Island, local school redevelopments, the City Rail Link and apartment law reform.

"There will always be things left on your list and there certainly are some - marine protection in the Hauraki Gulf, ferries to Waiheke.

"It's an extraordinary privilege to be an MP. As you walk through an airport, get into a taxi, people tell you stories they don't tell other people.

"I will miss the extraordinary power you have to make a difference in people's lives."

But she doesn't think she'll miss contributing in the corridors of power when, for example, important education reform is being debated.

"I might be fishing on the Barrier or going for a swim or catching up with my nieces or nephews. I think I'm more likely to be doing that."

She said she had already been emailed about "political to corporate" opportunities in the short time since her retirement became public.

"I always want to be helping people, so it's probably more likely to be charitable, but I'm going to take it slowly and enjoy not working so many hours and have a bit of time out.

"I'm looking forward to having a bit of a break, and spending a bit of time on the Barrier. That is my spiritual home, if you like.

"Because of the nature of breast cancer, I'm not at all worried about the future. I feel very at peace and very happy. I've had an amazing time in Parliament and contributed a lot, but now is the time for the next chapter."