"He's actually required to review that target under the Climate Change Response Act 2002 and he appears to have failed to have done that."
The second decision being challenged was the emissions reduction target set by the Government earlier this year, and which it will take to UN climate negotiations in Paris next month.
Ms Thomson argues the Government's Intended Nationally Determined Contribution (INDC) - reducing domestic greenhouse gas emissions by 11 percent below 1990 levels and 30 per cent below 2005 levels by 2030 - as "unreasonable and irrational" against the seriousness of the issue.
"The lawyers will be arguing that this level is a lot less than the scientific consensus [was arguing for]," she said.
"He hasn't taken into account the relevant factors when making this decision and over-all, it's really unreasonable."
As far as she was aware, this was the first case of its kind in New Zealand, although there had been many judicial review cases.
"I really hope that the Government will be forced to review its targets and set more ambitious ones," she said.
"I also hope it will result in them creating a real plan to deal with climate change issues as well."
Questioned on the lawsuit today, Mr Key described the action as a "joke". "If we are getting sued, I hate to think what is happening to the United States and Australia and other countries because actually... their climate change targets are lower than ours."
Mr Key said New Zealand was going to Paris with a "very credible target" that was "about middle of the park" - but more ambitious than that of the US, Australia and developing nations like China and India.
He added the Paris talks would not include legally binding options on governments.
"We are also committed to doing more and will do more - ultimately we need to find scientific answers to what's been happening."
A spokesman for Mr Groser said the minister was seeking legal advice from officials and not in a position to comment.
A Ministry for the Environment spokesperson said the ministry had not received a copy of the proceedings and was "not in a position to respond".
Law expert: student's case a strong one - partly
University of Otago public law lecturer Marcelo Rodriguez-Ferrere said the first part of Ms Thomson's argument was a strong one, although the second was much weaker.
"She's right in that section 225(3)(a) of the Climate Change Response Act 2002 appears to impose a mandatory duty on the Minister to review emissions targets every time the IPCC releases a report, and if the Minister hasn't, then that's a breach of this section and the Court could require the Minister to review the target," he said.
"I should note that the court does not have the ability to substitute its own view as to what the target should be, just the power to require the Minister to follow the appropriate process."
However, the second part did not appear to be as strong, he said. New Zealand's INDC was a provisional commitment tabled with the UN, which would be submitted at the Paris negotiations.
"This commitment is not governed by New Zealand domestic law and is instead part of New Zealand's international relations, which the court is very, very reluctant to intervene in."
Essentially, he said, it was very similar to any international negotiations conducted by the Government.
"The Government has a prerogative power to conduct and engage in those negotiations, and while Parliament has the final word in ratifying any subsequent agreement that arises from those negotiations, the negotiations themselves aren's subject to any particular legal oversight.
"Certainly, the Court cannot require the minister to set targets that are consistent with what the scientific consensus says needs to be done to avoid the disastrous consequences of climate change as there is no legal requirement on the Minister to do so."
Auckland University associate professor of law Ken Palmer was not aware of anything in the Climate Change Response Act that required a particular outcome from the Government.
"Further, unlike the Dutch situation, there is nothing in the New Zealand Constitution requiring the government to take action on climate change or protect the environment or to commit itself and others to certain reductions in green house gas emissions.
"Those targets are policy matters for the government, which could not be the subject of Court intervention on judicial review.
"The action may be a strategy for placing pressure on the Government but I would expect that it would not succeed."
New Zealand's climate target
Mr Groser has previously described New Zealand's contribution to emissions reductions as fair and ambitious, saying at the time of the target announcement that the Government needed to ensure it was achievable and to "avoid imposing unfair costs" on any particular sector or group of people.
Almost 80 per cent of our electricity was renewable already, and around half our emissions came from producing food for which there weren't yet cost-effective technologies to reduce emissions.
He added at the time investment in agricultural research was beginning to bear fruit and the cost of electric and plug-in hybrid vehicles was continuing to fall.
However, nearly all submitters who suggested emissions reduction goals in the Government's climate consultation wanted a target much more ambitious than that set.
An analysis by the Ministry for the Environment of submissions received during the Government's May to June consultation on what its post-2020 climate goals should be showed there was a strong call for ambitious target and leadership from the Government.
Many of the more than 15,000 submitters related taking ambitious action to protecting our international reputation.
The 10,900 submitters in the consultation who specified targets wanted the reduction target set at 40 per cent below 1990 levels, or a target of zero carbon by 2050.
The Berlin-based Carbon Action Tracker initiative has also rated New Zealand's 11 per cent reduction proposal as "inadequate" and claimed it could be achieved without the country taking any action to contain a growing level of emissions over the next 15 years.
The organisation claimed that if most other countries were to follow New Zealand's approach, global warming would exceed 3 or 4C, a world that would see oceans acidifying, coral reefs dissolving, sea levels rising rapidly, and more than 40 per cent species extinction.
But Victoria University climate change professor Dave Frame disagreed that the concept of global burden-sharing of emissions reductions could be seen or argued as a form of justice.
"In fact, if you cut global CO2 emissions from 2005 by 50 per cent by 2050, and can get to net zero emissions by 2100, then you're quite consistent with a 2C target, depending on what you do with other gases and parametric luck in terms of the climate system response," he said.
"Given that New Zealand's gazetted long-term target actually is 50 per cent reductions by 2050, if the rest of the world embarked on the same course as us, we would be acting pretty consistently with a 2C target."
He added 40 per cent reduction by 2030 had no special status.
"That view requires specific value judgements about how countries should share out emissions. In general these emissions recipes are abstractions which wrongly pretend that the allocation of a specific good amounts to a form of justice - but this view is nonsense.
"Even if you believe in global resource equality - a minority position, and an untenable one for a sovereign government - this entails equality over access to a basket of goods, not that each item within the basket should be equalized.
"So there's no obvious reason to slice emissions up according to a specific recipe."
According to present projections, the mean temperature in New Zealand could be 2C higher by the end of the century - and even between 3C and 4C higher if no action is taken to curb the world's carbon emissions.
Within the same period, sea level is expected to rise between 50cm and 100cm, leaving populations to adapt by either abandoning coasts and islands, changing infrastructure and coastal zones, or protecting areas with barriers or dykes.
