That's the view of protesters following Ms Ley releasing her decision late on Friday.

Her approval was the final step after the NSW Department of Planning, Industry and Environment, and the state's Independent Planning Commission ticked off on the project.

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This now clears the way for Wollongong Coal to extract 3.7 million tonnes of coal over five years from the Russell Vale site - with some of the mining occurring underneath areas of the Sydney water catchment area.

In explaining her decision Ms Ley said the extension did not pose any concerns about climate change.

"I have accepted the department's recommendation on the available evidence, that a decision to approve the proposed action would be unlikely to lead to an increase in global average surface temperatures," Ms Ley said.

"The proposed action is not likely to cause more coal to be consumed globally (and therefore more greenhouse gas emissions) than if the proposed action was not approved."

Nic Clyde from protest group Lock the Gate pointed out this was the first approval of a coal mine since the Federal Court found, in her position, she had a "duty of care" to protect Australian children from climate change harms.

"This is a terrible decision for Australian children, for the environment, and for more than five million people who rely on drinking water in the Greater Sydney catchment area," Mr Clyde said.

"Wollongong Coal's dire financial situation and rotten environmental history were grounds enough to reject this project - but instead Sussan Ley has rolled out the red carpet for this company.

"Greater Sydney is the only large metropolis in the world where coal mining is allowed beneath its drinking water catchment.

"Instead of approving this mine, we should be looking at how we can better prepare for a future where coal is no longer needed."

Stop Russell Vale Mine spokeswoman Renata Field criticised the minister's decision, which came just weeks after the UN's Intergovernmental Panel on Climate Change released their report stating escape from human-caused climate change is no longer possible..

"In the wake of the most catastrophic IPCC status report on climate change thus far, Minister Ley's decision is unconscionable," Ms Field said.

Members of the protest group visited the Russell Vale mine gates over the weekend to demonstrate their opposition to the approval.

"The Russell Vale expansion will generate an extra 11 million tonnes of greenhouse gas emissions, at a time when we should be transitioning rapidly away from coal," Ms Field said.

"We simply can't afford to let this project go ahead. It would also result in 34 coal trucks travelling through our residential areas every hour, generating extra noise and pollution."

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