New Zealanders appear to have experienced a groundswell of republicanism as a new poll shows nearly sixty per cent want a Kiwi head of state.

The poll shows 59 per cent of New Zealanders wanted to change the centuries-old system of Government, compared to 34 per cent who favoured the next British Monarch becoming King of New Zealand.

Undecided voters remained unchanged on seven per cent over the last three polls.

Monarchy NZ chairman Dr Sean Palmer.

New Zealand Republic chair and former deputy secretary of Foreign Affairs and Trade Peter Hamilton said he thought Brexit, the flag debate and the fact that the Queen was now 90 years old, all contributed to New Zealanders thinking seriously about this country's place in the world.

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But Monarchy New Zealand is questioning the integrity of the poll, carried out by Curia research on behalf of New Zealand Republic.

Of those who wanted to ouster the British Monarchy, only 44 per cent wanted to elect its replacement themselves.

A total of 15 per cent of respondents wanted the head of state appointed by a two-thirds majority in Parliament.

Monarchy New Zealand chair Dr Sean Palmer said it was flawed logic to combine those two groups and say 59 per cent supported a constitutional change.

"They've decided that the 15 per cent who voted for parliamentary appointment, would vote against the monarchy, if they didn't get their way.

"What if the offer that's on the table is it's appointed by a two thirds majority of Parliament? We'd know that 15 per cent would vote for that, but there's no indication that 44 per cent are also going to vote for that," he said.

"In fact it's more like that they're going to vote the other way."

Support for a New Zealand head of state had risen across every age group, over the last 16 months.

A total 76 per cent of 18 to 30-year-olds supported change in 2016, up from 59 per cent last year.

Support waned slightly, the older the age group got, but more than half of those aged 61 and over still supported change - 53 per cent.

Hamilton said everyone had a "tremendous respect for Queen Elizabeth", and she was "very much still on the throne".

"But we're going to have to face the fact that in 10 years or so, she won't be the monarch any longer."

Hamilton said a formal discussion should occur, but they did not need to take place immediately.

"I think the fact is this has showed changes across all age groups. I do think this means people are thinking about it, and when you ask them the question 'should a New Zealander be head of state', people think 'well why not?'

"It's a pretty obvious thing."

Palmer questioned the wording of the poll.

He said asking whether people preferred a New Zealander as head of state, rather than a member of the "British Monarchy" becoming the next King of New Zealand was loaded.

"It's the King or Queen of New Zealand. That is a fundamental constitutional distinction that is one of the founding principles of this country's independence."

Palmer said New Zealand had a "very successful" constitutional framework.

"And it's so deeply woven into the fabric of New Zealand society."

Asked if New Zealanders should have the chance "to aspire to be" the head of state, Palmer said the idea made him "nervous".

"There is the opportunity to aspire to the highest political office of the Prime Minister, there is the opportunity to aspire to be the Governor General.

"But I think really when we're talking about the ultimate power in a nation, which needs to be safeguarded for the public... I'm a little nervous about people who are desperate to become the supreme power in a country."

Prime Minister John Key has conceded a republic was eventually inevitable, but would not comment on the poll.

Labour leader Andrew Little said the surge in republicanism was odd.

"It is interesting there's been such a significant increase in such a short space of time.

"I guess the flag debate to some extent focused on issues of national identity. It certainly, for me the head of state is a vital element of our national identity.

"That might be partly an explanation for it, I can't think what else it would be."

But now was not the right time for a formal discussion.

"My view is the time to do it, is at the end of the reign of the current Monarch. I don't think we should preempt when that might be.

"But at some point we ought to have a good public debate about it, though right now there are other big priorities and it's not something I'd be wanting to climb into straight away."

Little said a New Zealander should be New Zealand's head of state, and a cross-parliamentary appointment would keep the role "above the fray", but that person had to have the confidence of the public.

Curia, also the Government's pollsters, carried out the poll in August.

Curia Director David Farrar is a former Councillor for the Republican Movement of New Zealand and a declared supporter of the New Zealand Head of State Campaign.

The landline poll was conducted over August 8 to 23, with a sample size of 1006. The margin of error was 3.2 per cent.