Labour says National's tax cuts plan is falling "to pieces" amid ongoing questions about whether the numbers stack up in its flagship policy.
On Sunday night, Newshub revealed National only sought independent advice on whether its foreign buyers' tax could break international law two days after the policy was released.
That's despite senior National MP Paul Goldsmith last week saying: "We can find a way through, other equivalent places like Australia and Vancouver have these taxes - and all we're trying to do there is gain a bit of extra revenue to give some relief to the squeezed middle."
National believed the foreign buyers' tax would raise $740 million to contribute to its $14.6 billion tax relief plan.
But Labour leader Chris Hipkins said the National Party's plan was falling "to pieces" and didn't "add up".
"The scrutiny is ramping up on those parties that have had a relatively easy run until now," he told AM.
"We've seen the National Party's policy starting to fall to pieces under scrutiny.
"Their tax plans don't add up, they want to sell $5 billion worth of Kiwi homes to offshore investors, they want New Zealanders to increase the amount of gambling they're doing and they want to put a tax on talent coming into the country," said Hipkins, referring the foreign buyers, offshore gambling and user-pays immigration levies National was proposing.
The Council of Trade Unions, while launching attack ads against National and leader Christopher Luxon this week, said the tax cuts plan was evidence the party was "seriously out of touch".
"National's plan under Christopher Luxon is short-sighted, it is not good economic management," union president Richard Wagstaff said.
Economist Shamubeel Eaqub told Newshub Nation on Saturday National's revenue estimates on its proposed foreign buyers' tax were "bullshit".
"Whoever's done the numbers and who's done the quality assurance really needs to have a really good look at this stuff because I don't believe they've got it right," he said.
But the person hoping to be New Zealand's next Finance Minister, National deputy Nicola Willis, said last week the party was confident in its costings.
"We had them externally verified and checked by Castalia consultants - they think all of our assumptions are reasonable [and] they agree with our modelling."
When asked by AM if she was certain there were no holes in the proposed foreign buyers' tax, Willis said: "Yes, we are sure."
National modelled its costings on how many sales it expected in the more expensive part of the housing market, Willis said.
"We're very confident… that the numbers we've put forward reflect the number of purchases you'd expect at that luxury end of the market and that will generate the revenue that we need to guarantee our tax reduction."