Bill English and Jacinda Ardern have clashed over housing, immigration and tax in the first campaign debate between the two leaders.

Questions put to each leader by moderator Mike Hosking included what a fair price to pay for a house is, if Kiwis wages are rising enough, and whether they will free-up access to medicinal cannabis.

English and Ardern were respectful of each other - each took the chance to cut in on certain topics. However, there were no moments when each talked over the other for any real length of time.

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A rare moment of agreement was on their shared concern at tensions escalating between the Trump administration and North Korea.

The debate on TV One came just an hour after the release of a sensational new poll in which Labour overtook National in the party vote, and Ardern bettered English in the preferred prime minister stakes.

The 1News Colmar Brunton poll showed Labour had risen 6 points to 43 per cent, while National had fallen three points to 41 per cent.

English opened by denying that his party was losing the race, saying the latest poll was "a bit out of line with" with National's own polls. Ardern refused to celebrate, saying the election was "not a done deal' and she was treating the figures with caution.

Tax and wages

Early on English attacked Labour's "vague and confusing" policies and talked up his party's commitment to tax cuts - telling Ardern her party would take $1000 a year off meatworkers in Horowhenua.

"People can't go shopping with your values ... every person in New Zealand who does not have children will be worse off under Labour."

Ardern returned serve later by saying while the economy was fairly robust, she was also aware that for two-thirds of New Zealanders, their pay increase last year did not keep up with the cost of living.

Productivity is 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.

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Clean rivers

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

English defended National's record on clean rivers after Hosking suggested 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."

The National leader warned a royalty on water would leader to disputes over ownership with Maori - something Ardern strongly disputed.

"How will you solve the issue? ... Bill has bumped this to a working group which is meant to be reporting in December."

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Housing

The debate's moderator Hosking pushed Ardern on whether New Zealanders would pay a capital gains tax under a Labour government.

She said Kiwis would "never" pay one on the family home. She said she was asking voters to "hear us out" on its tax proposals, which would be considered by a tax working group in Labour's first term.

Ardern went after English on National's record on housing - repeatedly asking English, "do we have a housing crisis?".

She reiterated Labour's plan to build 10,000 houses a year, and said an affordable selling rate was $400,000 to $600,000. When Hosking said that was "less than a builder could build it" for, she said it would be possible at scale.

She did not directly answer questions about whether she wanted house prices to fall. English claimed "speculation has been beaten" by getting more houses built with 200,000 in the pipeline. He disputed Hosking's statement that enough houses weren't being built.

Immigration

English questioned how Labour would build hundreds of thousands of houses while at the same time cutting immigration.

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Ardern confirmed her party's policy would see net migration numbers drop 20-30,000, but said that was through cutting international student numbers at low-level courses, and not everyone arriving in New Zealand was a builder or electrician.

After Hosking questioned whether low skilled migrants were keeping Kiwis out of jobs, English said, "someone has to pick the Kiwifruit, someone has to milk the cows".

Ardern responded: "if everyone coming to New Zealand was going to the regions we wouldn't be having this conversation".

World affairs

Ardern was asked about the recent decision to expand the NZDF's contribution to a mission in Afghanistan by three personnel. She thanked English for providing information to her on the matter but said as Prime Minister she would want to review further details currently not available to her - saying her position could be summarised as "hesitant".

On the Presidency of Donald Trump, English said it had proven to be unique and "interesting". Any request for NZ to join a future conflict would be assessed on its merits but "we do want to play our role in the world".

North Korea was the biggest threat to world stability, English said, and he is concerned the tension with the US was escalating to a point where misjudgements could be dangerous.

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"I agree with Bill on that last one," Ardern said.

Closing statements

Hosking asked what each had learned following their unexpected elevation to the leadership.

English said he had become even more motivated about the choice in this election - vagueness and uncertainty, or "building on our economic strength".

Ardern said she had "learnt how much you can squeeze into a day" and the importance of being yourself. She said she was convinced New Zealand can be a better country.