New Zealand First leader Winston Peters has confirmed he would only form a government with Labour if he's told the party's tax plans.
Labour has promised to establish a tax working group, if elected, and leader Jacinda Ardern has said she would wait for its recommendations before proposing any tax changes.
Mr Peters told Morning Report the party should tell him and the public its tax plans before the election.
"The public need to know the full ambit of this tax policy that Labour is talking about."
He said Labour was able to rule out various options, so it should be able to tell voters what it was considering adopting.
"If you are able to rule out what you are not going to take up, then surely you know what you will adopt in the future."
Mr Peters wanted assurances before any coalition talks.
"I can not deal with negotiations on the basis of 'some time in the future you may well know'."
Mr Peters preferred Ms Ardern's predecessor Andrew Little's pledge to put a working group's recommendations to voters before acting on them.
If, after the election, Labour had not spelled out the options "we would still need to know, with great specificity, where you are going to go with this tax policy".
"Otherwise we're signing up to a pig in a poke."
Mr Peters maintained his stance was not a negotiating tactic by New Zealand First - it was what he believed the public was entitled to know before the election.
"If they get away with not telling the public, nevertheless I'd want to know because we'd want to know what we're buying here and what we're getting ourselves in to, because it could be a calamity."
Ms Ardern has ruled out any tax changes that could affect the family home or the land under it but has said she was keeping an open mind about other options.
She said today she would not be giving Winston Peters any more details about her party's tax plans as part of any potential coalition negotiations.
"We've shared all of our expectations and plans around that working group with the public and I'll be sending exactly the same message to Winston.
"We've put it all out there for voters to see so that they know what we will do when they're in office and why we're doing it, which is to address our housing crisis and to bring fairness into our tax system."
She said the aim was to address New Zealand's housing crisis and bring fairness into the tax system.
Green Party leader James Shaw accused Mr Peters of grandstanding.
He said the New Zealand First leader had no right to attack Labour's plans until he was transparent about his own.
"Mr Peters has released about $10bn of election promises himself but he's given no indication about how he plans to pay for those - either by increasing revenue or cutting spending in other areas."