New Zealand heads to the polls tomorrow in one of the most uncertain election outcomes in memory, after one of the most gripping campaigns.

One thing is certain: either National or Labour could lead the next Government, no matter which party has the most seats after the votes are counted.

A suggestion by Prime Minister Bill English this week that the largest party gets the first chance to form a Government under New Zealand's constitutional arrangements is not correct, and he has walked back from that.

The largest party under MMP has led the Government but any bloc of parties making up more than 50 per cent of the Parliament can actually form the Government.

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Labour leader Jacinda Ardern believes Labour could lead a Government without necessarily holding more seats than National.

"Absolutely," she told the Herald yesterday.

She said the suggestion the biggest party had the right to form the government was an "assumption" by English, not a constitutional arrangement.

"You could have a lower party vote and still be in a position to form a Government," she said.

"I accept there is a perception and presumption but it is not a constitutional issue."

Ardern was made Labour leader eight weeks ago and the party rocketed up the polls until National's concerted attack on it reserving the right to impose new taxes next term without seeking a mandate at the 2020 election. The policy was eventually reversed.

After a series of contradictory poll results through the campaign by the two main pollsters, Colmar Brunton and Reid Research, the two are now in some alignment, with Labour and the Greens combined close to National.

Last night's Newshub Reid Research poll showed that New Zealand First could hold the balance of power and effectively choose which party or bloc led the Government.

Ardern did not want to comment on the suggestion that the Green Party supporting a Labour-New Zealand First coalition was more likely than New Zealand First supporting a Labour-Green coalition.

She said Labour had done some thinking about coalition scenarios "but everything waits really until post-election".

In 1996, New Zealand First held parallel negotiations with Labour and National over two months before deciding to go into a formal coalition with National.

In 2005 New Zealand First leader Winston Peters said he would deal first with the largest party, Labour, and he did a deal to provide confidence and supply.

Ardern made a call at the outset to run a positive campaign "and when your fortunes change, campaigning becomes a lot more enjoyable".

Her patience has been tested in the past two weeks over claims that Labour would increase tax - people will pay higher tax under Labour but because it would cancel National's tax cuts to boost working for families, not because it would raise tax. But that was not enough to dent her positivity: "I've enjoyed every moment of it. I probably anticipated there maybe would have been moments I wouldn't have but I genuinely can say I've enjoyed all of it."

Speaking to the Herald from his campaign bus, English said the campaign had been "more competitive" than he thought it would be but it had followed the pattern settled on from early in the year - "a battleground around the core issues around the economy and the ability to deal with the other issues people have been concerned about".

He said that as the issues around "personality" subsided, policy become more important and that suited him and his National team.

English did not think the aviation fuel crisis had harmed or helped National but had had a "neutral" effect.

If it had escalated into a problem for commuters then it could have been a bigger issue in the campaign.

He said if National was the biggest party he would start making the calls required on Sunday to form a stable government. In reference to his comments that the biggest party has the first go to form the government under New Zealand's constitutional arrangements, he said it was "just a reference to the way it has worked".

"It is no stronger than that.

"It is not claiming any greater role than that."

He said he had known Winston Peters for years and working with him had been challenging "but I understand, and I'm sure he does, the obligation to get on and make government work for New Zealand if that is what voters are after".