NSW Planning Minister Brad Hazzard has joined Rio Tinto in an appeal to the Supreme Court to allow an expansion of the Warkworth mine in the Hunter Valley.
The minister and Rio Tinto subsidiary Coal and Allied are appealing a decision by Chief Justice Brian Preston of the NSW Land and Environment Court to ban the expansion.
A department spokesman said the court's decision "may have implications that are broader than this particular development; in particular for the assessment of other mining projects".
"It is common practice and appropriate that, as custodian of the NSW planning system, the minister is represented in deliberations of this kind.
"The minister's representatives will not be arguing the merits of the Mount Thorley Warkworth development," he said.
Mr Hazzard approved the mine in early 2012, but Chief Justice Preston reversed that decision in April due to "significant, adverse, biological diversity, noise and dust, and social impacts of the project", after an application by residents.
Coal and Allied acting managing director Darren Yeates said overturning the minister's earlier decision had "put a question mark over the entire planning system in NSW".
"The court decided to overrule the decisions made by the authorities not because of any errors in law or procedure ... but because it disagreed with the outcome of a rigorous planning process that had determined the project was in the overall public interest," Mr Yeates said in a statement on Sunday.
Last week Rio Tinto chief executive Sam Walsh said banning the expansion could compromise the business's future.
The minister's appeal claims the judge had failed to give enough weight to a report by the department's director general, which found the project was in the public interest, had socioeconomic benefits, and that biodiversity offset packages were suitable.
Last week, Rio Tinto lodged its appeal against Chief Justice Preston's decision and was granted an expedited appeal in the NSW Supreme Court. A hearing has been scheduled for July 30.
In February last year, Mr Hazzard approved a 1271-hectare expansion of Rio Tinto's open-cut mining operations near Bulga, which the company claims will give it access to more thermal and semi-soft coking coal. It also asked for permission to keep operating the mine until 2031, 10 years after the present approval expiry date.
The planning department found that not granting the extension would prevent about $32 billion worth of coal being extracted from the Hunter mine.