Labour leader Jacinda Ardern has put her drop in the polls down to what she described as a "negative campaign" by National on Labour's tax policy.

Speaking in the final debate between Ardern and National leader Bill English on TV1, Ardern said she believed New Zealanders do want change despite Labour stalling in the latest 1 News Colmar Brunton poll and National taking a healthy lead again.

The debate was just an hour after the release of that poll which showed National had launched ahead of Labour again and up six points to 46 per cent - while Labour was down seven points to 37 per cent. Both would need NZ First to form a government.

Ardern said the poll showed National and Labour and the Greens (on eight percent) together were 'neck and neck.'

Ardern did not believe the drop was due to Labour's uncertain position on tax policy - or her own decision to back down and rule out introducing any new taxes until after 2020.

She said voters she met were more concerned with health and mental health than taxes.

However, English and National had mounted an "unfair" campaign on Labour's tax policies and that had an impact.

"There's no doubt the negative campaign from National has had an effect. That's not going to change the way I campaign."

English denied it was a negative campaign, saying it had every right to question and challenge Labour's policies. He denied his claims about Labour's income tax were misleading, saying Labour would have to legislate to restore income tax rates again.

However English was also cautious about the poll results, saying he believed the Labour and National blocs were "neck and neck".

"We want voters to consider very carefully the choice they are making on Saturday."

Ardern also agreed the two parties were closer than the poll showed.

"This election will be determined by turnout."

English said National's rise followed voters getting into the "tin tacks" of the campaign and weighing up what the two parties were offering.

He had predicted in a previous debate that the 'stardust' around the elevation of Jacinda Ardern as Labour leader and initial excitement at the start of the campaign would settle and voters would start to look more seriously at what was on offer.

The debate moderator Mike Hosking also asked Ardern about her plans to try to renegotiate free trade agreements such as the TPP and the agreement with South Korea to allow Labour to ban foreign property buyers.

Ardern said South Korea had sought a carve-out to stop New Zealanders buying property there and asked English why he had not done the same.

English replied: "We didn't think we'd be able to achieve something like that and it wasn't our policy anyway."

English said New Zealand was a small country which was open to trade and investment. Ardern said she believed it was simply because National had not cared. "Well, I care."

Ardern said nobody would ever sign a free trade agreement if there was no ability to renegotiate in the future.

Ardern was also asked about NZ First leader Winston Peters statement that while he supported charges on water bottling, he would not support water levies on the rural communities.

Ardern said she was committed to implementing the water charges anyway.

Asked if she accepted rural New Zealand was offside with Labour, Ardern said she believed there was unity on the issue of clean water ways. She said farmers were not the main target of the water charges policy.

"I targeted water bottlers. That was something New Zealanders wanted them to pay their fair share, but there is a flow on effect.

The vast majority of farmers are not affected by this policy."

Asked if the accumulation of the water tax, the Emissions Trading Scheme and the Green Party's nitrates levy amounted to an attack on the farming sector, Ardern said they were not all Labour policy. She believe it was possible to farm smarter.

"The division we've seen has been stoked during this campaign, but not by me.

When we lift our brand as a clean green country we all benefit from that brand - especially agriculture because they trade on that brand."

English said National and farmers had already put a lot of work into water quality. He said the water levy was a randomly applied levy and mechanisms such as nutrient allocations and work with farmers was a better option.

He said every New Zealander - including himself - wanted clean waterways.

He said a levy on water bottlers would be difficult to implement because it would put a price on water.