Mike Saxon from the Department of Planning and Environment (DPE) said tragically a banned organochlorine pesticide, Dieldrin, was confirmed in one flying fox and there are signs that others had also ingested a poison.

"At this stage we have not been able to identify any person responsible and we do not know if this was a deliberate or accidental poisoning," Mr Saxon said.

"We are continuing enquiries but regardless, this tragic incident highlights the horrible impact banned pesticides have on our native wildlife.

"Grey-headed flying foxes play a vital role in our environment pollinating our forests and dispersing our rainforest seeds. They also feed on fruit, including backyard fruit trees.

"We know this can frustrate gardeners but remind people that grey-headed flying foxes are listed as a threatened species in NSW. They are protected under the Biodiversity Conservation Act and it is an offence to harm them," Mr Saxon said.

DPE is partnering with the NSW Environment Protection Authority (EPA) to inform the community about risks associated with improper storage and use of pesticides.

Director of Regulatory Operations Cate Woods said the use of Dieldrin has been banned in Australia since 1987, noting that it can accumulate in native animals and livestock and contaminate soil for decades.

"We much prefer that people come forward and dispose of these chemicals or poisons correctly, rather than try to dispose of them another way that may end up harming our environment and wildlife," Ms Wood said.

DPE would like to thank the team at Wildlife Rescue South Coast, North Nowra Veterinary Hospital, Taronga Zoo and volunteers who helped recover and identify the cause of death for these flying foxes.