Environmental and community groups have joined forces in seeking an end to longwall coal mining under the Special Area of the Woronora drinking water catchment.
The groups called on the state government to place an immediate moratorium on existing mining, cancel any other permits and refuse future extraction plans.
"We further ask the government to confer the designation of nature reserve on this area," they said in a letter.
"Doing so would help preserve the important habitats, such as endangered coastal upland swamps that are vitally important for filtering and maintaining our water supply."
Signatories include Sutherland Shire Environment Centre, Nature Conservation Council, Greenpeace, Total Environment Centre, and the National Parks Association Southern Sydney Branch.
The letter was also signed by representatives of Woronora Valley Residents' Association, Bundeena Progress Association, North Cronulla Precinct Committee, Otford Community Association, Grays Point Progress Association, Georges River Environmental Alliance, Illawarra Local Aboriginal Land Council and the group POWA - Protect Our Water Alliance.
Before and after photos, taken in 2014 and 2019 in the Eastern Tributary of the Waratah Rivulet leading to Woronora Dam, have been used to illustrate "the damage being done by existing mining".
The later photos show greatly discoloured water.
The groups say water is seeping into bedrock cracks, releasing metal contaminants such as iron, manganese and aluminium, which is turning the water orange and green.
Special Areas around Sydney's drinking water catchments are mostly unspoilt bushland from which the general public is banned.
Chair of Sutherland Shire Environment Centre, Tassia Kolesnikow, said, "No other country appears to allow longwall coalmining under their publicly owned drinking water catchments".
"It doesn't make sense to risk our water supply in this way."
More than 9000 people signed a petition circulated by the environment centre, calling on the government not to approve a proposed extension of one operation.
In-principle approval for the proposal by Peabody Energy subsidiary Metropolitan Coal has been given, with Planning Minister Rob Stokes to make the final decision.
Catherine Reynolds, from the environment centre, said Peabody submitted plans for the next longwalls (LW305, 306, and 307).
"Two of these will run directly under Woronora reservoir," she said.
"The Planning Minister's office advised us the decision about whether to approve these will be made early in 2020."