Green Party leader James Shaw has hit out at New Zealand First leader Winston Peters for demanding to know Labour's tax plans, saying he needs to explain how he would fund his own election promises.

Shaw said Peters saying he would not go into coalition with Labour without knowing what its tax plans were was "the pot calling the kettle black".

But others have applauded Peters' call, including Revenue Minister Judith Collins and the Taxpayers Union.

Peters said he believed that Labour already knew what it wanted from a tax working group but was telling people it had not decided.

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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.

"You can't possibly mean to go into an election saying 'my tax policy will be decided by a committee'."

Peters, a former Treasurer, also criticised Labour's fiscal plan saying what while Steven Joyce's claim of a $11.7 billion plan was wrong, there was no money over the next three years for anything other than health, education and welfare.

Shaw said Peters needed to be held to the same standard as Labour and the Greens and have his policies fully costed and independently scrutinised.

He had promised at least $10 billion of new spending without saying what taxes he planned or what public services he would cut.

"Unless he is transparent with his own plans, he has no right to attack Labour for theirs," said Shaw.

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.

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

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Taxpayers' Union spokesman Jordan Williams said it was not often the group backed Peters on matters of policy "but in this case he is spot on".

Collins tweeted that it was not often she agreed with Winston Peters.

"Why is Labour hiding their plans for inheritance tax, gift duty, water tax, land tax, income tax hikes."