About 300 fish are estimated to have died at West Lakes in Adelaide due to poor water conditions and hot weather.
The Department of Primary Industries (PIRSA) investigated the issue after receiving several complaints from the public.
"Our water quality testing found very low levels of oxygen at the inlet of West Lakes so it looks like the tide has not been moving much at all," Aquatic Animal Health manager Doctor Shane Roberts said.
"[There's been] high water temperatures as well of 26 degrees Celsius and the water was also fairly murky."
West Lakes is fed by tidal flows and was constructed on a tidal swamp.
The local council has removed the dead fish but Dr Roberts said it was possible more fish could die on Friday.
"The tides aren't going to be moving much tomorrow and it's going to be fairly warm so we might see a few more fish die, he said.
SA Health has advised people fishing in the area to not eat dead or dying fish.
