Labour is demanding the coalition government explain how it will deliver its promised tax cuts.
Finance Minister Nicola Willis on Wednesday unveiled her mini-Budget, but it contained few details on how tax relief will be delivered and how much it will cost.
She is confident changes to tax thresholds will be rolled out responsibly next year.
"The advice we have received so far gives the government confidence that we can responsibly deliver the tax relief New Zealanders deserve," Willis said.
But Labour finance spokesperson Grant Robertson wants proof.
"No one is any the wiser as to how National will pay for its tax cuts."
Willis, however, said New Zealanders cared more about the size of the tax cut, rather than how it was delivered - something the government is yet to work out.
NationaI campaigned on shifting income tax brackets and expanding tax credits.
During negotiations, the ACT Party convinced National to consider other ways of delivering the same amount - or more.
ACT leader David Seymour said the government was committed to delivering "the level of tax relief that was in the National Party manifesto".
"We are all committed to doing it in the most efficient, fairest and simplest way possible.
"Obviously, going into a Budget we are taking a range of advice on how to do that," Seymour said.
National's election policy predicts changes to tax thresholds will cost just under $9 billion, over four years.
But, given the final makeup of the government's tax package is still unclear, so, too, is the final cost.
Willis said the government was already taking steps to pay for it.
"The mini-Budget contained seven and a half billion dollars of savings, as well as concrete actions to deliver the next range of savings that will support tax relief."
The government has committed to rolling out that relief next year.
But New Zealanders will need to wait until May's Budget to know the full cost and how it will be delivered.