New Zealand's opposition National party cannot explain how it plans to tax foreign property buyers, which appears in contravention of international tax treaties and free trade agreements.

Opposition Leader Chris Luxon launched his party's campaign for government on Sunday in Auckland ahead of the October 14 election.

However, he and finance spokeswoman Nicola Willis have come unstuck by a key plank of its tax policy.

On Wednesday, the pair announced it would raise $NZ3 billion ($A2.7 billion) over the next four years by reversing Labour's ban on foreigners buying homes in New Zealand.

The ban - which does not apply to Australians - would only be lifted for foreigners buying properties worth over $NZ2 million ($A1.8 million), who would also be whacked with a 15 per cent tax.

Several economists have joined Labour to attack the tax as not able to be implemented, given New Zealand's free trade and tax treaty obligations to treat citizens of several countries the same as Kiwis.

Mr Luxon insists it is workable.

"I am rock solid and confident in our assumptions around our plan," he said.

"We think there's a way through all of that. We think we can implement it ... we've had independent legal advice, we've validated it."

Finance Minister Grant Robertson said National needed to buy a new calculator.

"Experts and commentators across the board have pointed out that National's housing tax simply will not work," he said.

"Our international tax and trade agreements detail in black and white that many of the countries they would need to even get close to paying for it are excluded.

"They are hundreds of millions of dollars short, just in one aspect of their policy."

National's tax policy has formed one of Labour's attack lines, with Prime Minister Chris Hipkins using it in his campaign launch on Saturday.

Mr Hipkins said National are "hoping people won't notice that our homes and roads will be up for sale to overseas investors".

Mr Luxon insisted New Zealand needed to pull more offshore capital.

"It's important that we can attract foreign direct investment to New Zealand," he said.

"We are the most restrictive country in the (developed) world on foreign direct investment.

"Actually being able to open up the ability for foreigners to buy houses ... that's all good common sense."