The National Party could be breaching international treaties with its plan to let foreign buyers purchase New Zealand homes again.
It wants to allow overseas buyers to purchase high-end homes - and pay a 15 percent tax - to fund the party's income tax cuts.
But Labour's claiming the numbers are crumbling around them.
Nicola Willis is the architect of National's numbers.
"Labour loves tax like a shark loves blood, and it's time for National to sort it out," she told a crowd at National's campaign launch.
But one tax expert believes it could be Willis who needs to sort out her own tax plan.
"I think there is significant and probably much more risk than they are contemplating" said taxation law academic Craig Elliffe.
National wants to let overseas buyers purchase luxury New Zealand homes worth more than $2 million, but they'd have to pay a tax.
Elliffe said doing so could even put our free trade agreements at risk.
"It is very damaging to us from a foreign, international trade and international reputation perspective," said Elliffe.
Christopher Luxon is comfortable with National's plan.
"No disrespect, I'm not taking any lectures from Grant Robertson or anyone on the other side about trying to say there are holes in our plan because there isn't," he said.
The legal advice National's released is about trade agreements, not international double-tax treaties.
Asked if National had two separate pieces of advice regarding tax treaties and trade agreements, Luxon said: "We have got independent, expert advice on how to implement the 15 percent foreign buyer ban given our obligations internationally."
Labour says if we can't tax any of the countries we have double tax agreements with, there's big trouble with National's plan.
"It’s just fantasyland stuff," said Labour leader Chris Hipkins.
Labour's done its own costings by creating three scenarios to see how short National could be of the $3 billion they need from their tax plan.
The first scenario is the most generous. It assumes foreign buyer levels are the same as 2018 from the countries which we could tax. Without those countries, there would be a $1.3 billion dollar hole.
Scenario two is that the tax and $2 million threshold puts off half of those buyers, creating a $2.1 billion hole.
The last is the Vancouver example which saw a 500 percent reduction in foreign buyers after it implemented a tax. That could see a hole of $2.4 billion.
"The real message to National is just show us your numbers," said Hipkins.
Asked if he would release his costings, Luxon said: "Many other countries around the world do this."
National is under pressure now to show us the money.