The expansion, approved by the NSW Independent Planning Commission (IPC) in September, would make the Mount Pleasant operation the biggest open-cut coal mine in the state, with an annual production capacity of 21 million tonnes.
The extension would run until December 2048, deepening part of the open-cut mining area near the Upper Hunter town of Muswellbrook, to allow for the extraction of about 444 million tonnes of run-of-mine (ROM) coal over the life of the mine.
Local community organisation Denman Aberdeen Muswellbrook Scone Healthy Environment Group (DAMSHEG) said it was worried about the environmental, social and economic impacts of the expansion.
The group's head Wendy Wales says Muswellbrook already has "the worst air quality in NSW".
"Allowing Mount Pleasant to become the biggest open-cut coal mine in the state, directly upwind of town, is only going to make this dangerous health problem worse," she said.
The Australian Conservation Foundation this year ranked the small town as the third worst postcode in the country with polluted air, behind Mt Isa in Queensland and Newman in Western Australia
In a 56-page report released in September, the commission said it approved the site after receiving 960 public submissions, with nearly 700 supporting the project.
"The project represents a reasonable and responsible extension to the life of an existing coal mine ... and is sufficiently likely to result in positive economic benefits," it said.
The IPC said the economic windfall for Muswellbrook and surrounding local government areas would reach about $140 million.
However, DAMSHEG maintains the environmental devastation, including air quality, cannot be ignored.
The NSW Planning Ministry's own environmental assessment also noted the expansion could also impact Muswellbrook's drinking water supply.
"Climate change is already running away on us," Ms Wales said.
"Surely we have learned something from the catastrophic heatwaves, droughts, fires and floods in recent years, here and across the world."
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