An investigation has commenced after hundreds of dead carp and eels were found at Bushells Lagoon, north west of Sydney.
Concerns began in late December with reports of dead pelicans at Deep Lagoon, about half an hour north of Windsor, and later at Bushells Lagoon, the ABC reports.
"On Sunday I went out there to have a look at Bushells Lagoon and it seems that every carp and every eel in the lagoon is dead. I haven't seen one alive — fish or eel,” Richie Benson from the Hawkesbury Environment Network said.
"It smells really bad, it's rotten, it's going make the water really poisonous in the next couple of days especially after today, a [40 degrees Celsius] day."
While there is no official explanation for why the fish have died, Mr Benson suspects it was a combination of hot weather and the low water level.
Hawkesbury City Council have begun to investigate the cause and will commission a report in conjunction with the NSW Environment Protection Authority on how to better manage the area's lagoons and wetlands.