Eight Australian children took action against Environment Minister Sussan Ley last year, challenging an expansion of the Vickery coal mine in northern NSW.

The Whitehaven Coal-owned project north of Gunnedah was previously approved by NSW's Independent Planning Commission and would result in 100 million additional tonnes of carbon emissions.

The students began their activism by organising protests as part of School Strike 4 Climate, the network founded by Swedish teenager Greta Thunberg.

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The protests were their way of bringing the climate crisis to the attention of the public and saw this case as their chance to bring it to the attention of the courts, lead lawyer David Barnden told AAP.

"It is a historical day for climate law in Australia and it should have far reaching implications," he said.

Justice Mordecai Bromberg ruled in May, in what class action proponents have labelled a world-first judgment, that Ms Ley has a duty of reasonable care not to cause the children personal injury when exercising her power to approve or reject the extension.

He formalised the declaration on Thursday, also ordering the Commonwealth pay the legal costs of the students and their litigation guardian, Sister Brigid Arthur, an 86-year-old Catholic nun connected to the Brigidine Asylum Seekers Project.

Justice Bromberg's order says Ms Ley has a duty to exercise reasonable care "to avoid causing personal injury or death to persons who were under 18 years of age and ordinarily resident in Australia at the time of commencement of this proceeding arising from emissions of carbon dioxide into the earth's atmosphere".

It only applies when she exercises her legislative powers in relation to the Vickery Extension Project.

A spokesman for Ms Ley said "the Morrison government will review the judgment closely and assess all available options".

Mr Barnden said the reasons behind the declaration are important and could have broader implications on other fossil fuel projects in future.

"The court looked at the evidence on climate science, the evidence of future harm to children as a result of emissions that were expected from the mine and burning the coal from the mine, and those factors will be important for any future decision the minister makes with respect to fossil fuel projects," he said.

Justice Bromberg said the impact of the Vickery mine on global carbon dioxide emissions and average temperatures was "tiny but measurable".

"In my assessment that (climate change) risk is real - it may be remote but it is not farfetched or fanciful," he said.

Ms Ley is yet to make a decision on the Vickery expansion project and the current decision window is expected to expire on Friday, though it could be extended.

The children had also sought an injunction over the project, which was rejected by Justice Bromberg in May with him finding Ms Ley had not yet breached her duty of care.

Mr Barnden said once the minister makes a decision it's "entirely feasible" it could be open to a further challenge.

"The children are obviously thrilled with (the declaration) and the success in the court case but they will be waiting for the minister to make a decision on the extension project and we will see," he said.

Australian Associated Press