The public has been warned to avoid contact with the water in Jamison Creek, Wentworth Falls following the discovery of hundreds of dead and dying freshwater crayfish, native fish and other water bugs.
Preliminary surveys of the creek-line have not detected any visible signs of water pollution such as discolouration, odours or foam and the cause of the dead crayfish is yet to be determined by water testing for contaminants.
The NSW Department of Primary Industries Fisheries Division (DPI), the Environmental Protection Agency (EPA) and NPWS are currently investigating the problem with support from Blue Mountains City Council.
"The NSW DPI Fisheries and the EPA have the matter in hand and council is supporting the investigation in every way possible," said Blue Mountains Mayor Daniel Myles.
"Council officers will monitor the situation on a daily basis", he said.
"It is important that residents, especially children, do not attempt to touch any of the dead crayfish until more is known about the cause of the die-off."
The council is calling on anyone who may have observed anything being dumped or washed into the Jamison Creek between Friday and Saturday morning to contact Blue Mountains City Council on 4780-5000 or the EPA Pollution hotline on 131 555.
The problem extends from the Great Western Highway through to the waterfall at Wentworth Falls.