Under current carbon trading rules, as pine forests have grown, New Zealand has been able to gain credits for all the carbon stored, covering most of the gap between our emissions and our targets.
But the report authors point out that when the forests begin to be harvested from around 2020, New Zealand's so-called forestry "credit card" will be "maxed out" and the country will have to pay many of those credits back.
They say the proposed changes to forest accounting rules avoids the need to do so, as, under the new approach New Zealand would only receive credits for carbon stored in a new forest up until it reaches the long-term average carbon stock.
"For a pine plantation that would be after around 20 years," they say.
"If we applied this rule now, New Zealand would receive far fewer credits during the period up until 2020, but also wouldn't have to pay any back on harvesting."
Their main finding was that, up to 2020, New Zealand would claim 79 million credits - with a range of 65 to 90 million - from forests that were above their long-term average carbon stock.
"The excess emissions this allows equate to nearly one year's worth of New Zealand's gross emissions," they said.
"This approach will violate not only the forest accounting rules agreed under the Kyoto Protocol, but also the Paris Agreement's key principle of progression."
They concluded that, while there appeared to be some merit in the rule change, integrity required a corresponding increase in New Zealand's 2030 target of at least 25-30 per cent below 1990 levels.
Existing rules had bought New Zealand time to get its greenhouse gas emissions down, "but Government inaction has squandered that time", they said.
"Our emissions have continued to climb in excess of our targets and it will now take a huge effort to turn things around."
Labour climate change spokesperson Dr Megan Woods said it was "critical" the Government released all of the information so it could be seen whether the report's modelling was accurate.
"It is very worrying and very concerning if it is accurate, because this will be back-sliding on our 1990 level, when the world is about to commit to progression, more ambition and actually cutting emissions.
"This wouldn't be a problem if we'd kept up with planting, but we've maxed out the credit card without having made any of the payments that were required over the last eight years, in terms of replanting.
"This again just points to the need for an independent climate commission and carbon budgeting."
Climate Change Minister Paula Bennett was approached by the Herald for comment yesterday afternoon, but today a spokesperson from Bennett's office said questions had since been referred on to the Ministry for the Environment.
A ministry spokesperson said the ministry wouldn't be commenting on numbers or strategy as the rules hadn't been settled yet, but added New Zealand was committed to "ensuring that our approach to forestry has environmental integrity".
The ministry had taken a "broad approach" for how forestry would contribute to New Zealand's nationally determined contribution to the Paris agreement.
"While working on those details we are taking into account how the wider negotiations on forestry rules for the 2030s are playing out."
New Zealand and other countries would be negotiating the post-2030 forestry accounting rules and guidelines over the coming years.
"The Paris Agreement provides an opportunity for the global community to revisit the mechanisms for achieving emissions reductions to make them fairer, more workable and more transparent and robust than previously," the spokesperson said.
"New Zealand needs a plan for domestic action and the Government's announced three working groups to help with this."
