The National Party is refusing to release it's full tax policy modelling, which most economists are claiming will leave a half a billion dollar shortfall. 

Their concerns centre on the proposed foreign home buyers tax - which National is relying on to raise $740 million a year.

But using real estate data from Corelogic, the economists have calculated that revenue will fall short by some 60 to 70 percent.

The National Party's Deputy Leader and finance spokesperson Nicola Willis explained to Charlotte Cook why her party believes those projections are wrong.