The previous National-led government failed to take appropriate action over some of its climate change emissions targets, according to the High Court, but it won't face any consequences because it's no longer in power.

Justice Jillian Mallon released her decision on Thursday after Waikato law student Sarah Thomson took former Environment Minister Paula Bennett to court in June over alleged inadequate action to address emissions targets.

The High Court dismissed the judicial review. But in her written decision, Justice Mallon acknowledged that when the Intergovernmental Panel on Climate Change (IPCC) released its fifth assessment reports in 2014, the Government failed to undertake a satisfactory review of its 2050 emissions targets.

GED CANN/FAIRFAX NZ Sarah Thomson leaves Wellington High Court in June with legal team Sam Humphrey, Michael Heard, and Davey Salmon.

"However this cause of action has been overtaken by subsequent events," Justice Mallon said.

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Because National lost power at the last election, and Prime Minister Jacinda Ardern's new Labour-led government had already announced its intention to set a new 2050 target, court-ordered relief was ruled unnecessary.

Thomson said on Thursday that the ruling was a win, and she was more optimistic today about the country addressing its emissions.

"In some ways it's a win because the case brought a lot of attention to the issue and made, hopefully, more New Zealander's aware we aren't doing enough about it," she said.

"There are a lot of positives. The judge has laid out what should be mandatory considerations for a minister to take into account when making this kind of decision. That's really going to set guidelines for future ministers."

Government lawyers had argued the court was not in a position to make a judgement on the government's target setting, because they could not comprehend the many economic and social implications. But Justice Mallon did not agree.

One of Thomson's lawyers said on Thursday this was hugely significant, because it emphasised that all future decisions that were made in this area were susceptible for review by the courts.

"Notwithstanding the complex nature of these decisions, the courts retain a crucial oversight role to ensure that the decisions meet minimum standards of lawfulness and rationality."

On two other actions, which related to the 2030 Nationally Determined Contribution (NDC) emissions targets, Justice Mallon was not convinced the minister made any error for which the court could intervene.

"The international framework has been followed. It has not been demonstrated the NDC decision was outside the minister's power under this framework," she said.

"That is not to say another minister would have assessed the appropriate 2030 target in the same way and reached the same decision. Nor does it prevent New Zealand from doing more between now and 2030 than contemplated in its NDC decision.

"The international process envisages review and demonstrated progression by developed countries including New Zealand. Quite apart from the international process, New Zealand remains free to review its 2030 target (or any other target) as it considers appropriate.

"The community has elected a new Government and it is for that new Government to weigh the competing factors and to reach a view about the appropriate targets going forward. For these reasons the application for judicial review is dismissed."

Sarah Thomson said the ball was now in newly-minted Minister for Climate Change James Shaw's court to ensure targets were changed quickly.

"This is something we can't waste time on."