The mysterious deaths of pelicans, turtles and countless fish have alarmed residents north of Sydney who are now demanding action.
The dead and dying creatures have been found around Bushells Lagoon, near Wilberforce northwest of Sydney. The rotting carcasses are also creating an awful odour.
Concerned resident John Varley said one of the biggest wetlands on the Hawkesbury River is now in a "distressing state".
Most put it down to a combination of unusually high water temperatures, and pollution.
Xuela Sledge of Greater Sydney Landcare said the water quality has been decreasing continually over the past four or five months.
"The smaller amount of dilution of any chemicals in the water is certainly going to have an impact on the wildlife in the lagoon," Ms Sledge said.
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The devastation of aquatic life includes pelicans, native eels, water hens and turtles has worsened in the last week.
Whatever the cause is, the Environmental Protection Agency and the Hawkesbury council have yet to take responsibility. Locals say both organisations need to come to a mutual understanding before more aquatic life is wiped out.
Locals say they can't win though. Even once the water returns so will the carp - an unwanted pest in the region.