Climate activists say the government's pledge to halve emissions by 2030 vastly overstates the effect it will actually have.
It has boosted its international Paris Agreement pledge - or Nationally Determined Contribution (NDC) - to cut emissions from 30 percent ahead of the massive UN climate meeting.
To get there it will need to buy two-thirds of its reductions from other nations at the cost of about $1 billion a year.
The promise equates to a 41 percent reduction in real terms because of the way emissions are counted.
David Tong was a citizen member of the government's delegate in the 2018 UN climate negotiations, and is an Oil Change International spokesperson.
He acknowledged the updated pledge is heading in the right direction.
But he said screwy accounting by the government meant the real drop was less ambitious than it seemed.
Tong said the government was taking the 2005 starting point as the higher gross emissions figure - which includes everything the country's industry and people put into the atmosphere, and is then comparing it to 2030's proposed net reductions, which subtracts the savings from carbon trapped in our forests.
He said this made the pledge look rosier than it actually was.
"When you crunch the numbers on a net-to-net basis this isn't 41 percent, it's closer to 21 percent. On a gross-to-gross basis, it's closer to 31 percent.
"That means we're a lot worse than the UK, a lot worse than the EU, worse even than the EU or Japan."
'Target accounting' also questioned
James Every-Palmer QC from the Lawyers for Climate Action said there was another bit of what he sees as dodgy accounting.
He said the government was using what is called 'target accounting' which overstates the removal of carbon by forestry.
On his calculations, the actual reduction was less than half the government's headline figure of 50 percent.
He said the Lawyers for Climate Action were taking legal action against the Climate Change Commission about its advice to the government in relation to both the NDC and the domestic emissions budgets.
He said few countries use these accounting methods, although the government said it is actually quite common.
Climate Change Minister James Shaw said its accounting was sound and the amount of climate gases New Zealand was responsible for in 2030 would be half what it was now.
He said by pure coincidence, the point the country was in in the forestry cycle in 2005 meant the gross and net numbers were basically exactly the same.
Shaw said the Climate Change Commission recommended it continue using the gross-net approach as the two measures were close to identical, and that "most other countries" use it.