The Green Party has revised its tax policy after finding a calculation error that made the plan look $800 to $826 million better than it actually was, according to RNZ.

The mistake was in how the party treated extra funding for Inland Revenue. Instead of recording it as a cost — which was the right approach — the Greens counted it as money coming in. This made the overall revenue figures look larger than they should have been. The party quietly corrected the numbers once it spotted the problem.

The actual policy hasn't changed. The revised package still includes a 2.5 percent annual levy on net assets above $10 million and a 33 percent tax on inheritance or gifts received above $1 million. These two are the main money-raising parts of the Greens' wealth and inheritance tax plan, which has been a central part of how they've positioned themselves heading into the election.

What the error was matters as much as how big it was. Treating an IRD staffing cost as income is a basic modelling mistake — it made the revenue look bigger by counting a spending obligation as if it were actual money coming in. For a party whose main pitch is that it has credible alternatives on how government should spend money, fixing the error quietly rather than saying so straight away gets attention in the Press Gallery, whatever people think of the policy itself.

The size — somewhere between $800 million and $826 million — isn't small. To put it in perspective, that's bigger than the amount the current government has been working with each year in its operating budget. The real question now is whether the corrected numbers still show the policy bringing in more money than it costs.

Wealth taxes and inheritance levies have generally been popular in polls when people are asked in theory, but contested when it comes time to actually vote. The Greens have run this policy through at least one election cycle already. A costing mistake of this size, even once corrected, gives opponents something clear to point to. The timing might actually work in the party's favour — being well outside an election year means there's more breathing room to absorb the hit to their reputation, work through the numbers again, and put the policy back in front of voters with stronger ground underneath it.