Labour leader Jacinda Ardern says the election result is "not a done deal" despite another extraordinary jump in the polls which put her party in a position to govern in a coalition with NZ First.

The One News-Colmar Brunton poll released last night turned the election campaign on its head, placing Labour ahead of National for the first time in 11 years.

The poll, which also had Ardern has New Zealand's preferred Prime Minister, came immediately before the crucial first leader's debate between the Labour leader and National leader Bill English.

Ardern was not getting carried away, saying after the debate that she was treating the numbers with caution.

"It did surprise me. But I will make sure, and my team will make sure, that we take nothing for granted.

"We've seen such a dramatic change in the polls in the last three to four weeks that that drama can go in either direction."

English said the poll had not affected him because it did not match with his party's own polling.

"Of course you're always worried about the election result ... but we have confidence in our approach."

He suspected that recent allegations from NZ First leader Winston Peters that National leaked details of his superannuation payments may have had "some short-term effect".

The debate was a fairly even contest and no killer blows were landed.

English persistently attacked Labour's "vague and confusing" policies, especially its lack of detail on its water tax and its possible capital gains tax.

He talked up his party's commitment to tax cuts - telling Ardern her party would take $1000 a year off meatworkers in Horowhenua.

"Every person in New Zealand who does not have children will be worse off under Labour," he said.

Ardern's main lines of attack were National's record on housing and homelessness, and its failure to significantly to lift incomes.

While the economy was fairly robust, she said that for two-thirds of New Zealanders their pay increase last year did not keep up with the cost of living.

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Productivity was flat-lining, Ardern said, and with almost half of all jobs threatened by automation and other advances it was necessary to invest in people. Labour's free tertiary policy would do that.

Ardern also pleaded with voters to "hear us out" on its tax plans, saying many of them would be dealt with by a tax working group in Labour's first term.

Water was a key topic of debate. Ardern stood by Labour's plan to introduce a royalty on water, saying New Zealand's rivers were dying and it wasn't right that water bottlers were not paying. The effect on farmers and other businesses would be small, she said.

English defended National's record on clean rivers after moderator Mike Hosking National had been "caught with its pants down" on the issue.

The country was smart enough to produce quality food for the world and lift water standards, he said.

"It's not a trade-off, it's really important people understand that."

English later made his strongest statement so far on National's plans for freshwater pricing, which are being considered by a working group.

"We will not be going with a tax on water," he said. That appeared to be a shift from a position earlier in the year in which he did not rule out charging bottlers of freshwater.

Labour's latest poll result could be directly tied to Ardern's elevation to leader. The party has risen 19 percentage points in the month since Ardern took over from Andrew Little.

On the latest result, Labour would be able to form a government with NZ First alone - a much easier prospect than forming a three-way alliance with the Greens.

NZ First, which has been embroiled in a controversy about leader Winston Peters' superannuation payments, fell 2 points to 8 per cent,

The Green Party are just clinging to survival on 5 per cent following the resignation of co-leader Metiria Turei over a historical benefit scandal.

While that level of popularity would allow the Greens to return to Parliament, they would have fewer MPs and could be frozen out of government yet again.

Under Ardern, Labour has placed greater emphasis on core Green issues like climate change and cleaner rivers, possibly drawing away some of the Greens' support.

It has shown a willingness to spend some of the cash generated by a growing economy, promising to make tertiary education free and slash the price of GP visit for half the population.

The polling period also captured National's strong week, in which it announced plans to extend paid parental leave and give greater support to parents using IVF.