Nicola Willis says New Zealander's don't want spreadsheets as pressure mounts for National to release its foreign buyers tax costings.
It comes after a group of economists released a report claiming National's foreign buyers' levy had a significant fiscal hole.
National is proposing a 15 percent tax on the purchase of properties worth more than $2 million by foreign buyers to help pay for several tax cut initiatives, like shifting income tax brackets to account for inflation.
On Thursday, three leading economists modelled three scenarios. Remember National needs to raise - on average - $740 million a year.
The most generous scenario - which assumed a growth in housing stock and sales - found the policy would generate nearly $290 million a year, leaving National about $450 million short. Their most likely scenario was it making $212.7m - that leaves a $527m hole.
But on Friday, National's finance spokesperson Nicola Willis told AM the economists have "underestimated how much demand there will be for homes from foreign buyers".
Willis said the economists have taken a very "pessimistic view" where they believe there will be very few foreign buyers - but her party disagrees.
"We have done a method that projects forward how many we would expect to sell with a tax on and without Australian and Singaporean buyers being subject to the tax. And we've come up with a number that's less than half of what was there previously, because we accept that we're no longer going to be selling homes under $2 million. So that's a pretty conservative number."
She said National's view has been checked by external economic advisers and "they agree with our view".
"There's an old saying, 'You get six economists, you'll get seven different opinions in the room'."
AM's Ryan Bridge continued to push Willis on why her party wouldn't release its costings and just put the issue to bed.
"We do think it's really important to be open and transparent with New Zealanders. That's why in our tax plan, we detail our methodology for this, we have a table that goes through the costings of every part of our plan in every year."
When asked if Willis would commit to release the costings, and if not, if she was scared, she told AM "I don't think what New Zealanders want from National is Excel spreadsheets".
Willis said she's never known a political party to release spreadsheets and she "won't be the one who starts".
The party's finance spokesperson stood by her plan, which has been put together "really conservatively and carefully".
"In the first year of our tax plan, we book more than $500 million more in savings and extra revenues than we are planning to pay out in tax cuts. That's how conservatively we've put it together."
Willis told AM whenever there was a choice between a conservative costing and a least conservative one, they've always gone conservative.
"New Zealanders want to know we can responsibly deliver this plan, that's fair for them to want that assurance and we can give that assurance to them."
If elected to govern, Willis promises her party will deliver its tax plan without borrowing.
Watch the video above for more.