"Public confidence takes a knock when government agencies that are critical of the project are heard behind closed doors,'' the party's mining spokeswoman Lee Rhiannon said.

Environmental groups claim the project has huge ramifications for the Woronora and Georges rivers.

Planned by BHP Billiton subsidiary Illawarra Coal, it would involve mining under 220 square kilometres for the next 30 years.

Sharyn Cullis, the secretary of the Georges River environmental alliance, said BHP's coal mining proposal threatened at least 55 swamps and 47 streams.

"The clean freshwater provided by the myriad of tiny streams flowing out of the swampy landscape is vital for our rivers' ecological health,'' she said.

"It offsets all of the polluted urban stormwater downstream.

"Expect coal mining consequences to flow downstream and affect us all. Locally our swimming, fishing, boating and waterskiing will be spoilt. Drinking water catchments will be damaged as well.

"Only the state government can stop this unfolding environmental tragedy for our rivers.''

The Sydney Morning Herald reported on Monday that memos it had obtained revealed government agencies had told the Planning Assessment Commission that Sydney's drinking water was likely to be damaged if the plan went ahead.

Departmental bureaucrats had only allowed the evidence to be provided to the commission in private and BHP Billiton had also made its presentation behind closed doors.

"Originally BHP admitted in their environmental assessment that longwall mining leads to riverbed cracking, water loss, pollution spikes, iron staining, swamp draining and cliff falls,'' Ms Cullis said.

"Yet at a recent public hearing, BHP were not required to give evidence, nor publicly answer questions.''

The Herald said criticism of the proposal had come from the Sydney Catchment Authority and the Department of Environment, Climate Change and Water.

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