Even a political scientist who has studied New Zealand politics for more than 35 years was surprised by John Key's resignation today.

Raymond Miller, of the University of Auckland, said the announcement was "a shock to everyone, particularly to people inside the party".

"Here was John Key in his prime deciding to call it quits. I think we were expecting him to go in his fourth term, between 2017 and 2020.

"When you're polling at 50 per cent after eight years, you're doing pretty darn well."

National faced a more uncertain future without Key at the helm, he said.

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

Key will go down in the history books as "the most popular Prime Minister in modern history".

"John Key appeals in part because of his seeming ordinaryness. There's something about him. He's able to communicate well with people. He's also friendly with the public.

"He has moved the party into the centre politically. He's very pragmatic, followed public opinion. You've got to take the public with you, and he's clearly done that."

Tristram Clayton discusses John Key's departure from politics with political scientist Raymond Miller. Photo / NZH Focus
Tristram Clayton discusses John Key's departure from politics with political scientist Raymond Miller. Photo / NZH Focus

Key's resignation was good news for Labour, Miller said.

"They've always seen John Key as the personification of the National government. They've seen him as their greatest threat to success.

"So I'm sure they'll be looking now and thinking, 'well some of the pressure will be off' and next year there's always a chance that a Labour-Green party arrangement could result in them winning the election."

With Key vacating the top job, it was likely to go to Deputy Prime Minister Bill English, Miller predicted.

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"I'd be somewhat surprised if it's going to be contested, but I could be wrong. If it is contested I'm sure there are people like for instance Steven Joyce and Paula Bennett and perhaps even Judith Collins, who might put their hand up as potential leaders.

"If they have Bill English as Prime Minister they may well go for a much younger, newer deputy. Someone like Simon Bridges for instance."

Miller said Paula Bennett may also put her hand up as deputy.