Winston Peters has kept up his criticism of Labour's tax plans - accusing Labour of creating confusion despite having "nine years to get its tax story straight".

The New Zealand First leader has focused on recent comments by Labour leader Jacinda Ardern, in which she estimated the cost of the proposed royalty on water could be between 3 to 6 cents per kg of milk solids produced.

Today, Peters issued a press release questioning Labour's calculations about how much revenue a royalty would generate, and said more definite details were needed.

"This is yet more confusion from a party that has had nine years to get its tax story straight," Peters said.

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"That is why New Zealand First wants to know what this policy is all about. Because tax by nature is definitive, not vague and uncertain.

"The only thing we learned from the Labour leader is that they won't tax the water being used by the family home. That's a good start so can we please have all the other details."

The Labour Party has promised a royalty on water if it is in Government, saying users in rural areas would pay around 2c per 1000 litres and the money would be given to councils and iwi to restore waterways. The charge would be set after consultation with industry and iwi.

The amount of water used by farms varies depending on several factors - location, type of farm, and whether they use irrigation. Labour's tax would only apply to water sources from waters and aquifers (but not town supplies).

Peters has said he needed much more clarity on the water royalty and changes to be considered by a tax working group, including a possible capital gains tax, before he could support a Labour Government.

He said he believed that Labour already knew what it wanted from a tax working group but was telling people it had not decided. The voters deserved to know before the election and if that did not happen then he would expect to be told during any post-election negotiations.

Labour's policy in 2011 and 2014 to introduce a capital gains tax was replaced by former leader Andrew Little with the promise of a tax working group to look at the whole tax issue but to seek a mandate for any changes at the 2020 election.

Ardern has kept the tax working group but wants to implement changes without seeking a mandate.

On Monday there will be a Federated Farmers-organised protest in Morrinsville - Ardern's hometown - against "continued attacks" on rural New Zealand. The proposed water royalty has generated controversy and been attacked by National, with Bill English accusing Labour of creating a rural-urban divide.

Ardern has labelled such comments scare-mongering, and promised to work with farmers so any charge was not onerous.

A Herald-ZB Kantar TNS online survey released last month shows that 70 per cent of people agree that commercial water users should pay a royalty to help fund the clean-up of waterways. Just 19 per cent of the 1000 respondents oppose the idea, while the remainder are unsure.

The Green Party, the Maori Party and The Opportunities Party also want a price on water. New Zealand First supports a charge for commercial water bottlers, but not primary producers.

'Polls are all over the place'

Last night's Newshub Reid Research poll was bad news for NZ First and Labour. The poll had National up four points from a week ago to 47.3 per cent, with NZ First slipping to 6 per cent. On the results, National would be able to govern alone.

That is in stark contrast from the 1 News Colmar Brunton poll last Thursday night which had Labour steady on 43 per cent from the week before, with National slipping to 39.

In an email to supporters last night, Peters dismissed the polling as not accurate, and said National would not believe them.

"In their desperation, Mr English and National are now making big promises. And, remarkably, they're using money we didn't think existed until now."

Peters is speaking at public events in Whangamata and Whitianga today.