Labour's tax policy is putting strain on its potential support partners in an alternative Government, New Zealand First and the Greens.

And Prime Minister Bill English yesterday doubled down on his attacks on the policy, following criticism of Labour's position by New Zealand First leader Winston Peters.

But his comments drew criticism too from Greens co-leader James Shaw who accused Peters of "grandstanding" and said it was "the pot calling the kettle black."

He had made $10 billion in promises without saying how he would fund them.

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Peters said he believed leader Jacinda Ardern already knew what she wanted from a tax working group because she could so clearly rule out areas it must not go - the family home and any land tax under the family home.

She should tell voters before the election and if she did not do that, she would have to tell him in any post-election negotiations.

But she gave that prospect short shrift, at the Newstalk ZB Leaders' Breakfast.

" We have shared all of our expectations and plans around that working group with the public and I will be sending exactly the same message to Winston.

"We've put it all out there for voters to see so that they will know what we will do when we are in office and why we are doing it."

Ardern also dismissed suggestions by English that she was looking at a whole range of taxes - death duties, land tax, gift duties, inheritance tax.

"Most of these things have the same effect. His is listing them as though we are doing everything. He is wrong. You are wrong," she told ZB's Larry Williams.

She said the tax working group, which could include former Labour Finance Minister Sir Michael Cullen, would be asked to address two issues: stalling rates of home ownership and unfairness in the tax system.

Following Peters' comments, English told reporters that Labour "shouldn't just tell Winston Peters - it should tell the New Zealand public which of its seven taxes it is proposing are going to apply, what the rates are going to be."

An inheritance tax would have widespread impact, and would affect every family where the parents died and leave a house.

"What they are asking the public to do is vote for a committee in this election to decide one of the most critical issues for our economic success and that is how the tax system works.

"They are asking New Zealanders to hand them their ATM card and they'll give it back in a year when they've decided how much to spend."