The NSW Environment Protection Authority (EPA) investigation was launched after more than 50 Corellas were found dead or seriously injured in Manilla in March.

Laboratory testing by authorities revealed the native birds may have been grazing on a treated crop like sorghum, which poisoned them.

A spokesperson from the EPA said traces of pesticides often used as coatings on seeds prior to planting a crop were found in samples taken from the affected birds.

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"Although these are not highly toxic to birds, in some combinations they can be," the spokesperson said.

The EPA had initial concerns the bird kill event could have been a deliberate act, but the findings of the investigation indicate the poisoning was not intentional.

However, the source of the tainted grain and where the birds may have consumed it could not be determined by the EPA.

The environmental authority warned residents to take care when disposing of old and unwanted seed that has been coated in pesticides, fungicides and insecticides, because they can harm wildlife and other animals who might consume them.

The bird kill was reported on March 31 by a local wildlife carer after more than 50 Corellas were discovered dead or seriously injured on the north-eastern side of town near the Viaduct bridge.

The EPA has asked the community to report suspected animal poisonings to the organisation's environment line on 131 555 or call WIRES on 1300 094 737.

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