A review of New Zealand's position on an international climate change league table has seen us relegated to a position equal with cellar dwellers China, the United States and Australia.

The multi-agency Climate Action Tracker used the latest United Nations information to review New Zealand's 10-20 per cent 2020 emissions target, later dropping our ranking from "medium" to "inadequate".

Prime Minister John Key said this week - before leaving for Copenhagen - that the "knowledgeable" climate watch group had assessed New Zealand as being in the "medium" group of countries, which showed New Zealand was in "pretty good shape".

Climate negotiations minister Tim Groser had also referred to the "medium" ranking in Copenhagen this week saying it showed New Zealand was doing its share.

But the climate action tracker website said the way New Zealand wanted to use forestry credits would boost the amount of greenhouse gases it could emit without breaching the 10-20 per cent target - making it easier to meet its targets and entitling it to a worse "effort" rating.

The previous assessment assumed the rules around forest credits would stay the same.

The Sustainability Council's Simon Terry reviewed the rankings last weekend and concluded that New Zealand's "medium" rating was wrong.

Yesterday he said there were other problems with the target that had not been picked up.

"What climate tracker has yet to report is that there are essentially no gains for the environment from New Zealand's pledge on current plans, just the temporary storage of emissions in trees that are to be chopped down in the 2020s," he said.

"Those trees time-shift the emissions reduction burden to another generation (but) they do not reduce emissions long term."

Forest credits are important to New Zealand and the rules being negotiated in Copenhagen could make the difference between committing to a 10-20 per cent emissions cut or a lower target.