Environment authorities are investigating the death of "thousands" of fish after they washed up on the shores of several locations in Western Sydney.
The most recent fish kill event took place under Rydalmere Bridge on the Parramatta River on February 6.
Chris Bahjat, who regularly fishes along the Parramatta River, described the incident as unique.
"I've seen a few fish kills but not like this," the 33-year-old plumber from Merrylands said.
"Literally, thousands of thousands of dead fish, everywhere you walk, a metre apart.
"You fish with the system all your life, you watch water quality slowly improve, and then it's disappointing when you see that many fish dying."
Mr Bahjat said he saw fish of all sizes belly up, including bream, flathead, whiting, luderick, herring, puffer fish and prawns.
Parramatta councillor Kellie Darley described the river as littered and 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 NSW Environmental Protection Authority (EPA) began clearing some of the dead fish yesterday but the ABC found leftover fish and fish parts were producing strong odours as they began to rot.
The EPA's director regulatory operations, James Goodwin, confirmed the agency had received numerous reports in recent days across several sites including Rydalmere, Ermington and Homebush Bay.
Mr Goodwin said it was distressing to see about 4,000 fish dead at one site alone.
Last week, on February 3, they were alerted to the first incident at Haslams Creek in Sydney Olympic Park, a body of water that connects to Parramatta River via Homebush Bay.
The EPA's early investigations suggest that low oxygen levels — likely caused by a high-density storm — were behind the events and not 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," Mr Goodwin said.
"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."
At Haslams Creek, chemical pollution was ruled out as a possible cause.
Mr Goodwin said water samples showed conditions there had improved and the sighting of "some large fish" was "promising".
Building a picture of the oxygen levels at different parts of the river to prevent another event was the EPA's main focus, Mr Goodin said.
The clean-up is ongoing, with Parramatta Council working with other agencies around the Rydalmere area.
The Department of Primary Industries said fish kills are more common in summer and follow sudden changes in temperatures.
Professor Joy Becker, an expert in Aquatic Animal Health and Production at the University of Sydney, said the kills occurred because the water conditions didn't strike the right balance to sustain fish life.
"When the water quality drops, it compromises the physiology of the fish, and if the decline in water quality is too much or too prolonged, it will lead to death," she said.
"A lack of oxygen or sudden and severe changes in water temperatures are often the most common causes, but also contamination with chemicals are common - or a combination of all of these factors."
