The collapse of a wall of coal waste at the Clarence Colliery into part of the Wollangambe River is expected to take "many weeks" to clean up, the NSW Environment Protection Agency said.
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The EPA described the pollution of the river near the Blue Mountains town of Lithgow as a "serious environmental incident" involving tonnes of so-called coal fines. Investigators spent Friday probing the extent of the damage after the spill a day earlier.
"The Wollangambe River is our greatest concern and determining the impacts to the river is a priority," Gary Whytcross, an EPA director, said in statement.
The EPA on Thursday described the region as "a very sensitive environment", with the Wollangambe flowing into the Greater Blue Mountains World Heritage two kilometres downstream of the mine.
The agency has ordered the mine's owner, Centennial Coal, to build silt fences between the mine site and the river to prevent further damage. The company's clean-up activities "are expected to take many weeks given the extent of the incident and the difficult terrain in which the material has been deposited", the EPA said.
Katie Brassil, executive general manager for Centennial's external affairs, said the incident involved "the overflow of material from a holding cell".
"The site has now been stabilised and the spill contained," Ms Brassil said in a statement. "Our immediate priority is to minimise any impact to the environment. Establishing how and why this incident has occurred will be part of an extensive investigation."
