A spokesman for the Department of Planning and Infrastructure said the "cross appeal" documents had been lodged with the NSW Court of Appeal on Friday.

"The [Warkworth extension] decision . . . may have implications that are broader than this particular development, in particular for the assessment of other mining projects," the spokesman said.

Documents viewed by the Newcastle Herald reveal the grounds on which the government says the chief judge of the Land and Environment Court, Brian Preston, "erred in law" in refusing the Warkworth extension project.

The government appeal says Justice Preston gave insufficient weight to departmental reports in favour of the mine.

These reports related to the project's environmental offsets package, its socio-economic benefits and its public interest benefit.

Coal & Allied also filed an amended notice of appeal on Friday, adding six new grounds to an appeal that contests more than 30 points in Justice Preston's April 15 decision.

Coal & Allied's acting managing director Darren Yeates welcomed the government's intervention, saying the verdict "put a question mark over the entire planning system in NSW".

A Coal & Allied spokesman confirmed the company had met Premier Barry O'Farrell over the court decision, and confirmed it was circulating petitions in favour of the project to go to state Parliament.

Bulga-Milbrodale Progress Association spokesman John Krey said the group was disappointed in the government's action.

"When they didn't file notice of appeal immediately we were starting to hope that maybe they didn't agree with Coal & Allied but obviously now that's not the case," Mr Krey said.

The Coal & Allied spokesman acknowledged the company wanted to mine an area it had pledged to protect.

But it was offering "significantly greater" biodiversity offsets and would rehabilitate and protect eight times as much land as would be disturbed in the Warkworth extension.

The appeal is set down for hearing in the NSW Court of Appeal from July 30.