Prime Minister John Key says there is no secret scandal behind his decision to quit, saying he wanted to rewrite the rule book on leadership departures and do so on his own terms.

Key announced yesterday he would step down as Prime Minister and National Party leader next week and leave Parliament next year.

He predicted theories would abound about the "real" reason for his decision but told the Herald there was nothing untoward or sinister in it.

"The marriage is rock solid, the health is absolutely tickety boo. Give the doctor a ring."

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He said it was not at the behest of his wife, Bronagh.

"She's been amazingly supportive and if I really wanted to stay for a fourth term, she'd back me. But I just feel it has been a decade of nights alone for her, and it is the right time for me to come home."

He said his family had borne the cost of his political career, including intrusion in the lives of his children, Stephie and Max, as they grew into adults.

Max Key posted a photo of the family on social media and a message saying while it had not always been a smooth journey, he was proud of his father: "You're the best role model I could have asked for and my hero. Love you Dad."

The resignation came as a shock to Key's staff, his Cabinet colleagues and National MPs - as well as to the Opposition. It forced Labour leader Andrew Little to delay a trip to India for the second time - he had earlier postponed it because of the Kaikoura earthquake.

National has been setting up to win a fourth term and Key's departure could have a massive impact.

University of Auckland political scientist Dr Raymond Miller said National faced a more uncertain future without him.

"A lot of spice, a lot of speculation now. We can't be too sure of how National will do minus John Key."

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Bill English, with Prime Minister John Key, walking to the House to deliver his Budget 2016 speech. Photo / Mark Mitchell
Bill English, with Prime Minister John Key, walking to the House to deliver his Budget 2016 speech. Photo / Mark Mitchell

Key acknowledged his own "brand" was a factor in National's success. However, he believed he left at a point when National was polling highly enough and had a strong enough record to win a fourth term without him.

"In a way, no modern day Prime Minister, post-war, has left on their own terms. They've always been thrown out or had a coup or lost. And I had an opportunity to be the Prime Minister that transitions to the new team and that the country can absorb that and we can be living proof of a new way of running the rule book. That was why I did it.

"The timing is never going to be perfect. But if I went in three years' time, people would say 'you can make history and do five terms'. There's never a perfect time."

He said he had watched other leaders who stayed on too long.

"You've always got the feeling they just regretted it a bit, that somehow the public that had supported them all of a sudden turned on them and they felt angry about that.

"I didn't want to define, in my own mind, my political career by somehow feeling as if the public had been ungrateful to me or something."

CNN's website carries the news of John Key's surprise resignation, which has had an international impact.
CNN's website carries the news of John Key's surprise resignation, which has had an international impact.

Former Prime Minister Helen Clark tweeted her best wishes to Key, who had supported her unsuccessful bid to be UN Secretary-General.

After eight years as Prime Minister, Key is one of the longest-lasting leaders on the international stage and his decision received international coverage.

It featured prominently on CNN and the BBC as well as news sites across Australia.

Several of his fellow leaders also tweeted an acknowledgment - including former Australian PMs Julia Gillard and Tony Abbott as well as current PM Malcolm Turnbull, who has described Key as a role model.

Key said he made his final decision after a trip to New York in September.

Other than family, he told only Deputy Prime Minister Bill English. His staff and Cabinet colleagues were told yesterday morning and the rest of caucus just before he announced it publicly.

He said he had long said he was not a career politician "and I was right". Key said he was likely to return to the commercial world on boards both domestic and international. He would also hit the speaking circuit internationally, spend more time at their house in Maui and travelling with Bronagh.

He and Bronagh would remain living in New Zealand but would trade in their Parnell house for a smaller home at some point.