The Government has been collecting carbon credits from the country's forestry stock since the 1990s.

The Government has been accused of "playing accounting games" to achieve future carbon targets.

In the most recent Morgan Foundation report, general manager Geoff Simmons said the Government planned to alter the carbon-accounting rules around forestry just as the country prepared to harvest the lion's share of its forestry stock.

By doing so, the Government would effectively wipe a year's worth of emissions off its books without reducing any carbon outputs, Simmons said.

Morgan Foundation general manager Geoff Simmons.

Since the 1990s the government has been collecting carbon credits from forests, which act as a sink, trapping carbon from the air and storing it in the ground.

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Simmons said these credits would have to be paid back with a forest's harvesting, but a proposed change in accounting might abolish those repayments, provided the forests were replanted.

"What the Government is proposing moving to is an averaging approach," Simmons said.

"That would be fine if we applied it now, then we would get fewer credits and wouldn't have to pay them back.

"But what the Government is preparing to do is to make the change in 2020. We get all the credits up to then and then don't have to pay them back. It's the best of both worlds."

That would make future targets far easier to achieve..

The 2030 targets are set at 11 per cent below 1990 greenhouse gas emission levels. The foundation recommended these be adjusted to 30 per cent if the accounting changes go ahead.

To predict how the new accounting rules would affect targets, researcher Paul Young said the foundation had created its own models after the Government refused to release its projections.

The models predicted New Zealand could claim an additional 79 million carbon credits with the change, thereby erasing 79m tonnes of greenhouse gas emissions.

The "opportunistic tactics" could compromise New Zealand's ability to campaign on the world stage, Young said.

"We are quite worried about the precedent this sets, because it could open up to scope for a lot more countries to try and hide their emissions through creative accounting."

A spokeswoman for the Ministry for the Environment said the Paris Agreement allowed countries to consider how to reduce emissions throughout the 2020s as the international community developed rules for the period after 2030.

"The report agrees with us that averaging is a sensible approach, but doesn't acknowledge that we are still developing the details of the forestry rules and accounting system we're proposing," she said.

"Whatever the final outcome, our approach will be internationally peer-reviewed."