Dead fish and eels have been found washed up at a creek in Sydney's north-west, prompting an investigation by the Environment Protection Authority.

The decomposing specimens were spotted at at Devlins Creek, under the Cheltenham Causeway aqueduct.

Local resident and spokesman for the Save Beecroft Cheltenham Alliance Michael Bianchino said he saw fish and eels "embedded in sludge and oil", at the bottom of Sutherland Road, Cheltenham.

"My gut feeling is that it is a combination of dry conditions, oil sediment runoff and low water levels, meaning there is less oxygen in the water, so it stagnates and you get toxic build up."Mr Bianchino questioned whether it was "coincidental" that the dead species have appeared within three metres of the construction site for the Epping to Thornleigh Third Track (ETTT).