The New South Wales Government's amended State Environmental Planning Policy (SEPP) officially comes into effect today, potentially altering the outcome of Rio Tinto's plans to expand its mine, near Bulga.

The modification replaces changes made in 2013 to prioritise the economic benefits of mining projects, after the Mount Thorley Warkworth proposal was rejected on social and environmental grounds.

But the mining giant remains optimistic the new policy will have a limited impact on the strength of their latest application.

Jeff Smith, executive director of the NSW Environmental Defenders Office, said the decision rests with the Planning Assessment Commission (PAC).

"It'll be up to the PAC to make a decision balancing those economic, social and environmental considerations," he said.

"I haven't been a party to Rio Tinto's economic and modelling, or the proposal they have at hand.

"However, it would seem to be essentially the same proposal that was rejected by the chief judge of the Land and Environment Court."

The merits of the project will be the topic of the PAC hearing in Singleton next week.

Mr Smith said the change is good news.

"We would say that at least it balances and levels the playing field, and gives the community the best possible shot of getting an acceptable outcome," he said.