A restricted insecticide has been found to be the cause of a mass poisoning of hundreds of native birds near Dubbo in central west NSW.
The NSW Environment Protection Authority (EPA) has investigated the deaths since they were first reported on the Talbragar River in the Troy Reserve area more than a week ago.
On Tuesday, tests carried out on dead birds found the insecticide fenthion to be the likely cause of death.
Fenthion is commonly used to eradicate fruit flies and non-native bird pests.
It has been heavily restricted in the fruit industry since late 2013.
A NSW EPA spokeswoman told AAP the authority would be involved in cleaning up the insecticide if the source was found.
Despite concerns about water contamination from carcasses in the Talbragar River, samples from the nearby Macquarie River contained no traces of fenthion, she said.
Wildlife rescue group WIRES has been cleaning up the bodies of mostly little corellas, sulphur-crested cockatoos and galahs since last week.
It raised concerns about the poison spreading up the food chain after two seriously ill whistling kites were found at a site of about 80 dead and sick birds on Monday.
The group's Dubbo chairwoman, Ann Mara, told AAP volunteers had picked up about 550 birds but put the actual death toll about 700.
Only 30 birds have been saved from the popular breeding site.
Ms Mara says the area's flock of little corellas has "been just about destroyed".
Fenthion was first flagged for review in 1994.
A 2005 Australian Pesticides and Veterinary Medicines Authority assessment led to home-use products being discontinued and bird control products restricted to trained personnel.
Two fenthion products meant for use in the fruit industry were suspended in 2012.
