Reports say thousands of dead fish have washed up along NSW’s Parramatta River. 

Environment authorities have been sent in to investigate a mass fish kill after they washed up on the shores of several locations in Western Sydney.

The NSW Environmental Protection Authority (EPA) have begun clearing up some of the dead fish, but there have been complaints that the leftover fish is beginning to smell. 

Parramatta councillor Kellie Darley says the river is under stress.

“It's a shocking sight, with all sorts of dead fish … along the whole Rydalmere foreshore,” she said in a Facebook post dated February 6.

“Given the tides, it's unlikely that they will be washed back into the river so it's going to get stinky along here.”

The EPA says its initial investigations suggest low oxygen levels - likely caused by a high-density storm - have reduced water quality.

“These events occur from time to time. They’re not particularly common but it’s a combination of factors [such as] a build up [of[ organic matter like leaf litter in drains and that sort of thing,” EPA director of regulatory operations, James Goodwin says.

“A high-intensity storm can come and wash that into the waterways. This then starts to decompose and suck the oxygen out of the water.

“And that presents problems for the fish in terms of getting their own oxygen.”