The Sustainability Council says last month's Budget reveals the country will still face a liability under the Kyoto Protocol despite the introduction of the Emissions Trading Scheme (ETS).
The council says taxpayers will owe at least $1 billion to meet New Zealand's commitment to reduce greenhouse gases.
On Wednesday, council executive director Simon Terry and economist Geoff Bertram released a book called The Carbon Challenge that argues the scheme doesn't go far enough in cutting emissions.
Mr Terry says present estimates of the country's liability are based on the credits received from forests planted before 1990.
But he says when those forests are cut down - a point conceded in the Budget - the credits must be paid back.
More debt if price goes up
Even after all the revenue from the ETS is collected, Mr Terry says, New Zealand is still $1.1 billion in debt on the Government's carbon price - and if that price went as high as $100 a tonne the debt could be $5.7 billion.
He says households will bear half the cost of the scheme in the first five years, while accounting for just 20% of the country's emissions.
But the Minister Responsible for Climate Change Issues, Nick Smith, says that the figures in the book are speculative and that there's still a lot of uncertainty about just what liability the country might face.
Farmers and the ACT party are continuing to campaign against the scheme, which comes into effect on 1 July.