Justice Mordecai Bromberg on Thursday made official his ruling that Ms Ley owed the duty to all Australian children when exercising her legislative decision-making powers, regarding the Vickery Coal Mine in northern NSW.

Eight children took action against Ms Ley last year, challenging an expansion to the Whitehaven Coal-owned project north of Gunnedah.

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

"After carefully considering the judgment, the minster has formed the view there are grounds on which to appeal," a spokesman said on Friday.

A formal appeal is now expected to be lodged with the court.

The appeal decision has been described as "pretty embarrassing" by 17-year-old Anjali Sharma, one of the teens involved in the case.

"Instead of doing her job of safeguarding our future, (Ms Ley) is prepared to spend public money fighting for her right to make climate change worse, harm the environment and risk the injury and death of Australian children," she said.

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".

While it applies only to her decision-making regarding the Vickery expansion project, David Barnden, who represented the teens in the lawsuit, said it could have much broader effect.

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

"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."

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, although 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.

Australian Associated Press