Winston Peters has set out a firm new condition of going into government with Labour - it must tell him what its tax plans are.
New Zealand First could not support any Labour government without knowing its true intentions, he told the Herald.
Peters said he believed Labour already knew what it wanted from a proposed tax working group but was instead saying it had not decided.
"I've got more experience than nearly all of them put together, with respect, lining up for these [post-election] discussions.
"And I'd expect to know, yes. As the people who trust Winston Peters would expect him to know."
In the same way, Peters said he wanted more clarity from Labour about how its proposed water royalty would be set, and assurances from National that Maori would not be given rights over fresh water.
Peters compared Ardern's rise to that of French President Emmanuel Macron, but claimed an uncritical media had played a crucial part.
"You are not asking the questions. You can't possibly mean to go into an election saying, 'My tax policy will be decided by a committee, and I am very sincere about that'. One needs to know what we are talking about ... that should be fatal to a party's chances. And we need to know."
Ardern has said there will be no decisions until after a yet-to-be-established expert group made recommendations on tax reform during Labour's first term.
That is a change from the position held by previous leader Andrew Little, who said if a capital gains tax was recommended by the working group, he would implement one only after taking such a proposal to the electorate at the next election.
Ardern says that is too long to wait, given the pressing need to tackle the housing crisis that is locking New Zealanders out of owning a home.
She has made clear a capital gains tax or land tax would not be applied to the family home, and has said income taxes won't increase.
Peters said those exclusions suggested Labour has a more complete plan for taxation.
"They know what it is. But they are not saying so. Because that's the only way you could intelligently rule out parts of the tax policy you have got a group set up to look at.
"Because you know what you don't want now."
Green Party leader James Shaw launched the party's climate change policy in Auckland yesterday and told supporters it was now not a question of whether Labour would win, but who it would invite into government.
"If you don't want Winston Peters holding Labour over a barrel, I am asking you to give your party vote to the Greens."
Speaking at the Stuff leaders debate on Thursday, Ardern said the Greens would still be her first phone call should she be in a position to form a Government. Yesterday she was asked if she would expect to negotiate with NZ First as a Labour-Greens bloc or just as Labour.
"I haven't set any expectations around that," she said.
Ardern said a Labour Government would set up the tax working group early on.
"[But] I expect they will want a good amount of time ... I'd want to give them at least a year."
Asked if former Deputy Prime Minister Sir Michael Cullen would be on the group, Labour finance spokesman Grant Robertson said: "He is certainly a man who knows a lot about those issues."
Peters said Ardern's rise and its "inexplicable" media attention had made it a difficult campaign.
Asked if he had seen anything like Ardern's impact on the campaign in his political career, Peters pointed to recent elections overseas.
"I've seen it before. I saw it in the United States, I saw it in the UK, and I saw it, in particular, in France in [now French President] Macron's campaign."
He said he expected the campaign to return to where it was in July before Ardern took over, with National and Labour's support levelling off. Smaller parties including the Maori Party and the Greens were in deep trouble - "you have a flush-out coming".
Peters talked of a "next generation" SuperGold card at a rally on the North Shore on Saturday attended by about 1000 supporters.
Ardern attracted about as many students recently at Waikato University, but Peters said his supporters were reliable. "The people who are writing about a youth quake may well be blowing in the wind. Because a youth quake only happens if people ... turn up. And they won't."