THE NSW Environment Protection Authority says it is determined to ''improve'' polluted run-off from a coal mine into the Georges River, south of Sydney, but conceded it probably wouldn't be able to stop it happening entirely.

The government has known about polluted discharges from the West Cliff coal mine near Appin, which is operated by a subsidiary of BHP Billiton, since 2002. Despite a long series of correspondence with the company, the government has not yet pressed them to stop the river pollution.

The company has not been fined or penalised, because it operates under an environmental protection licence that does not require limits on the polluting substances being discharged.

The contaminated water from the mine contains a range of heavy metals such as zinc, copper, nickel and aluminium at levels above healthy guidelines, plus coal ash and some arsenic, and it has damaged the river ecosystem for 15 kilometres downstream.

''In this particular case there has been a long process and a complicated process, and one where we probably would have liked to see things happen faster,'' said the chairman of the EPA, Barry Buffier, who visited the mine site yesterday. ''It's not an acutely toxic waterway, but we do acknowledge that the discharges have had an effect on the aquatic ecosystem.''