New Zealand's next Prime Minister: Who the MPs are backing

Support is growing among the National Party for Bill English to become the next Prime Minister after the shock resignation of John Key.

Key himself backed his deputy and finance minister for the top job during the announcement of his retirement on Monday.

But English is yet to announce he will seek the nomination, saying he would speak to the party and his family before making a decision.

He has known about Key's departure since September but said during a press conference just hours after the announcement that it was news to the rest of the caucus.

Members are expected to come together on December 12 to choose their next leader, while Key will tender his official resignation later that day.

Ministers Michael Woodhouse and Nathan Guy publicly offered their support for Mr English on Tuesday, as did backbencher Nuk Korako.

Backbenchers Mark Mitchell, Alfred Ngaro and David Bennett are staying mum on their choices - but Bennett says Key's view will have sway, the Herald's Claire Trevett reports.

She said Guy wouldn't say who he was backing for the deupty PM's job.

Corrections Minister Judith Collins said on Monday she had not ruled out seeking the leadership.

Former party leader Don Brash says he believes Collins has what it takes and would have his support.

While he ordained English as his successor, Key says whoever the National caucus choose to replace him will have his unequivocal support.

He said announcing who he would be voting for was not him trying to tell the rest of the caucus what they should do.

"They need to make their own call and it's their future and they'll back the leadership team that's right for them," he told Newshub.

"They will have my 110 per cent support. I would back whoever is the leader.

"In the end the caucus decides.

In 2002, English led National to its worst ever general election defeat, with just 21 per cent of the vote.

But Key said he had all the attributes to lead the party and the country after working closely with him over the last 10 years.

"There's a lot more than you see - everyone's style is different.

"It's really what's underneath the hood that matters. He's a thoroughly decent human being and would be a great prime minister."

Key told TV3 on Tuesday morning that he would speak to the Queen later in the day to inform her of his decision to stand down.

Meanwhile, NZ First leader Winston Peters believes Key's resignation came as the three-term leader loses touch with Kiwis.

While most party leaders have been diplomatic about Mr Key's surprise move, Peters, who has been in parliament through eight different prime ministers since 1978, is more blunt.

When Key's decision was announced on Monday, he was quick to fire off a statement saying there were "hidden economic reasons" behind it.

"John Key was beginning to look like he wasn't hearing people, that he was getting out of touch," he told Newshub on Tuesday morning.

"John Key's style was wearing a bit thin against their own private lives, their mortgages, lack of housing, homelessness all that sort of stuff."

He noted last time a prime minister went out on their own terms was National's Keith Holyoake in 1972.

"You've got to hand it to the guy, he was a teflon man. No matter what went wrong nothing seemed to stick with him, and that's a very lucky characteristic and a very likeable one if you're a National Party voter.

"He had some extraordinarily good luck, including for the most part an incompetent opposition."

Peters picked Deputy Prime Minister Bill English to be Key's replacement simply because the PM had nominated him. If National's caucus snubbed Key's choice it would be "astonishing", Peters said.

-AAP