John Key's stunning announcement that he is standing down has turned politics on its head and almost certainly reduced National's chances of winning a fourth term.

His personal popularity throughout his term in office was a main factor in his party's election successes, and it will surely miss him when it goes to the country next year.

However much Mr Key says the party is well positioned to win again, he has given Labour's Andrew Little a much better chance of becoming the next prime minister.

Mr Key has endorsed his deputy, Bill English, to succeed him.

English was leader of the opposition when National suffered a devastating loss in 2002, and he was rolled by Don Brash.

Mr Key insists the political world was different then, the cycle was running against English and now it will be running with him.

He could be right, but National's task has just got a lot harder.

It would be good for the party if English puts his hand up and isn't challenged, that way there won't be blood on the floor and no wounds to heal.

If he is challenged, Steven Joyce and Judith Collins are the obvious ones to do it, with Amy Adams a possibility.

And if English becomes prime minister, Joyce would be the likely deputy and minister of finance.

Mr Key was right to announce his decision now.

He couldn't have fought the next election pretending he was going to run a full fourth term, and then quit part way through.

As he said, he has been honest with New Zealand throughout his term and there was no way he was going to change now.

Mr Key spent nearly 40 minutes of his press conference explaining why he was standing down, saying he had never been a career politician and it was time for a change.

He said his wife Bronagh didn't give him an ultimatum, and would have backed him all the way if he had decided to run for a fourth term.

Mr Key has been an outstanding prime minister, although the opposition would differ on that.

He was the main reason National retained its level of support through three elections, and his departure leaves a gulf that's going to be hard to fill.

Malcolm Turnbull says his New Zealand counterpart is leaving the country's economy in a very strong state.

The prime minister says Mr Key has been a great role model as a reforming PM, who's won and retained strong public support for his economic reform.

Labor leader Bill Shorten says Mr Key is a civilised conservative and has been a good friend to Australia.

- with Reuters