The mysterious deaths of 300 seagulls that dropped from the sky in July in two beachside suburbs south of Perth may never be explained.

The state's Department of Environment and Conservation confirmed yesterday that two months of exhaustive investigations, including dozens of autopsies, interstate forensic testing and pollution inspections at nearby businesses, had failed to identify a cause.

The deaths closed the popular Woodman Point beach for more than two weeks amid fears of a threat to human health.

Almost 150 seagulls were found dead on the beach on July 21. The death toll reached 230 after three days and 282 a week later. No other bird species were affected.

A department spokeswoman said extensive tests had eliminated everything from heavy metals, organophosphates and alphachloralose (used to cull feral pigeons), through to exotic diseases such as avian influenza, Newcastle disease and West Nile virus as possible causes.

Without other leads, she said, the department was likely to "close the book" on the investigation next week.

The seagull deaths came just six months after a serious health scare in Western Australia that only came to light after thousands of birds dropped from the sky in the holiday town of Esperance, 800km southeast of Perth. The deaths affected a range of species.

Hundreds of children and adults were later found to have potentially harmful blood lead levels caused by lead carbonate, which was being exported through the Esperance port, being blown around the town.

Faced with a lack of answers from the Government, the Cockburn City Council opted last month to reopen the Woodman Point beach.