Labour's keeping the heat on National over its centrepiece tax policy - suggesting its promise is collapsing faster than a house of cards.

The party's election tax plan would shift income brackets for the so-called squeezed middle, using $14.6 billion found by reprioritising spending and adding taxes.

Minister Grant Robertson says that makes huge assumptions on how much would be raised, and is likely to be impacted by international treaties.

Finance spokesperson Nicola Willis insists the plan's fully costed.

"If you're buying a house on the other side of the world, then you're actually interested in that luxury part of the market. We've had a bit of house price inflation since then too."

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