Bayside Council says it has removed more than a dozen large dead fish found floating in ponds in Bicentennial Park at Rockdale.
A council spokeswoman said council staff had removed the dead fish and other rubbish from the ponds. The council was alerted after receiving calls from the public.
The council spokeswoman said the fish were feral carp and "not suitable for natural waterways".
A number of social media users recently took to Facebook to post photos of the dead fish and seek explanations of what had happened.
Daniel Ferreira Gebbran wrote on the Brighton-Le-Sands 2216 Facebook page on November 18 that he and his wife had across "the terrible scene" of at least a dozen large fish lying dead on the grass.
"We knew there were fish but we had no clue they were this big, and this many," he wrote.
"The staff who was working removing the fish, told us they simply turned out dead overnight."
He wrote that they were "super worried since there have been multiple animal poisoning reports across Sydney recently".
Another Facebook user Jo O'Rourke wrote on the Rockdale Residents Unite Facebook page on November 17 that she had "just been to Bicentennial Park and loads of fish are dead in the pond".
She posted images of the dead fish floating in water covered in green algae.
She later posted: "Called the council and they are aware of the situation and have an environmental officer investigating".
The council spokeswoman said the parks on either side of the ponds were once rubbish tips, and leaching into the ponds had occurred ever since they were capped over in the early 1980s.
"The leaching increases after rain and when it is warm, as it has been for the last few weeks, this creates ideal conditions for algae to grow, in this case duck weed," she said.
"The duck weed may also be causing de-oxygenation of the pond water at night and/or as it decomposes, or there could also have been a temperature inversion where the cold low-oxygen water from the deep part of the pond mixes with the surface water.
"This may kill off larger fish but smaller fish can often survive.
"Council is looking into what actions may be implemented in the future to prevent this issue from reoccurring."