A chemical used to control insects and non-native pest birds is likely to blame for the deaths of hundreds of wild birds near Dubbo in the state's central west, the NSW Environment Protection Authority said.

As many as 700 birds, mostly little correlas, galahs and sulphur-crested cockatoos, have been found dead over the past fortnight in a two-kilometre radius of Troy Reserve on the Talbragar River, said Ann Mara, chairwoman of the WIRES wildlife rescue group.

A volunteer plucks out a dead little corella from waters near Dubbo. Credit:Louise Donges, Daily Liberal

The EPA said testing of samples from the dead birds indicates fenthion, a pesticide commonly used to kill insects, spiders and birds such as starlings, is the most likely cause of the deaths.

"At this point, we do not have evidence as to how the birds came into contact with the insecticide but we are continuing to investigate, Gary Davey, director of the north branch of the EPA said.