National 'sure' there are no holes in tax cut policy, leading economist Shamubeel Eaqub unconvinced

Nicola Willis is backing her tax policy, despite claims from economists and Labour that National's calculations are wrong.

The National Party finance spokesperson told AM on Thursday she disagreed with allegations her numbers didn't add up.

"Yes, we are sure," Willis said during an interview with host Ryan Bridge when asked if she was certain there were no holes in National's foreign property buyers tax to help pay for its policy.

In its announcement on Thursday, National said it would partially lift the ban on foreign buyers - introduced by Labour in 2018 - but slap them with a tax. National estimated $740 million would be generated by allowing foreign buyers to purchase properties worth more than $2m with a 15 percent tax on top.

But leading economist Shamubeel Eaqub has called National's costings "a little bit optimistic".

"If you think about where foreigners were buying houses before the ban came in, they were buying properties not just in Queenstown and Auckland and mansions and whatnot, they're also buying rental properties in Rotorua and other places," Eaqub said. "I'd be very surprised if their numbers… really came to pass in the next 12 months.

"The reality is, if you look at the history of it in New Zealand, we weren't selling that many houses to foreigners in the first place - so I suspect the numbers are a little bit optimistic."

However, Willis said there'd been a large amount of house price growth in New Zealand since the foreign buyers' ban was introduced.

Willis said National the party modelled its costings on how many sales it expected in the more expensive part of the housing market.

"We're confident in our 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," she told AM. "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.

"These [properties] will be in the upper quartile so what we've modelled is that the areas where foreign buyers have tended to purchase in the past - which is Queenstown and Auckland - we've said that actually, in Queenstown, around one in four houses are above that $2m mark and in Auckland around one in eight.

"We're confident that these numbers stack up."