The Department of Environment and Conservation says birds that include ibises, silver gulls, ducks and ravens were found dead or convulsing within less than a mile radius of the landfill site.
The agency determined that the pwoisoning was due to the pesticide fenthion, which is used by industry and around the home.
It is not known if it was used to deliberately kill the birds or if someone dumped large quantities of it at the landfill, where the birds later came in contact with it.
The mass killings occurred less than a year after 200 gulls died mysteriously only a few miles away.
Two years ago, thousands of birds fell from the skies over the coastal town of Esperance, about 400 miles to the southeast, apparently poisoned by lead carbonate.