The latest chapter in a court battle between the Climate Change Commission and a group of lawyers kicks off in the Court of Appeal on Tuesday.
The lawyers want the government to do more on climate change. They have challenged advice from the Climate Commission, arguing it led New Zealand to adopt targets that are too weak to meet the goal of staying inside 1.5C of fossil-fuelled heating.
The commission was formed in 2019 as an independent expert group to advise the government of the day on the road to net zero emissions. Its advice helped set the country's first set of emissions budgets - sinking lids on climate pollution each year - as well as the international target for 2021-2030 under the Paris Agreement.
But the lawyers' group - Lawyers for Climate Action - argues crucial advice to the government was wrong.
The group has challenged two things: the emissions budgets the commission recommended, and its advice on whether New Zealand's international climate target was consistent with keeping the planet under 1.5C - the goal written into the Zero Carbon Act.
The crux of the case was how much New Zealand needed to cut emissions by in the next seven years.
In a short statement, commission chair Rod Carr said its advice "must be based on delivering an economically and technically achievable transition to net zero in an equitable and inclusive way by 2050".
He said the commission expected its advice to be "robustly assessed".
The root of much of the disagreement is pine trees, specifically pre-1990s plantation pines that were planted for their wood, long before the country adopted climate targets.
New Zealand's huge swathes of commercial plantation pines create large swings in net emissions, or the actual emissions the atmosphere sees from cars, factories, farms and so on after subtracting the carbon dioxide sucked in by the country's trees.
Depending on whether the trees are in a harvest cycle or a growth phase, these pre-1990s forests can make the country's annual emissions total very high or very low, even if underlying fossil fuel emissions and permanent forests had not changed.
The commission and the government got around having their tallies swing like this by leaving these trees off their Paris Agreement carbon accounting, which they used to work out how New Zealand was doing against its international targets.
They still want to count some other, newer forests, however, so they use a modified kind of accounting - a version of net emissions, but not the true total the atmosphere receives in a year.
The High Court ruled in favour of the Climate Commission, finding the commission was allowed to take into account New Zealand's special circumstances (historic pine planting) when finding a path for emissions compatible with 1.5C.
While that way of doing the maths could make it look like New Zealand was doing better than it was, the climate change minister understood the position and was not misled by the advice, the court found.
The lawyers appealed. They said because of the way the target was set, true net emissions could be higher in 2030 than they were in 2010 and still meet the Paris target.
They argued New Zealand's target cannot be consistent with what was needed to stay under 1.5C, as laid out by the top global climate science body, the Intergovernmental Panel on Climate Change.
The lawyers' group will also challenge other High Court findings, including one that the 1.5C target in the Zero Carbon Act is merely "aspirational".
The hearing starts in the Court of Appeal in Wellington on Tuesday and is due to finish on Thursday.