About 1,000 greater crested terns have been found dead on three remote South Australian islands, two of which sit in the largest breeding colony of the endangered Australian sea lion. The birds were found on Wednesday and the state government announced the three mass mortality events on Thursday.
The sites are Troubridge Island off the Yorke Peninsula, and North Page and South Page, the two islets between Kangaroo Island and the mainland known together as the Pages. Premier Peter Malinauskas told the South Australian parliament the birds were presumed to have died of H5N1 and that testing to confirm it was still running. "The loss of more than 1,000 birds is deeply concerning, and the state government is treating these events with the utmost seriousness," he said.
No Australian sea lion has tested positive for the virus. What has been confirmed is H5 in birds at and around the colonies, including greater crested terns on Kangaroo Island and at Seal Bay, which holds about five per cent of the world population of the species. South Australia's own count of confirmed H5 cases stood at 177. A little penguin from Kangaroo Island is at the CSIRO Australian Centre for Disease Preparedness at Geelong awaiting a result.
There are fewer than 12,000 Australian sea lions and they live nowhere else, with about 85 per cent of them in South Australia. They breed in dense colonies, which is the condition under which the virus has done its worst everywhere it has reached marine mammals. Rachael Gray, a University of Sydney veterinarian who works on Kangaroo Island and has seen H5N1 in Antarctica, said the location is what matters. "It's very concerning when it's at Australian sea lion colonies," she said.
The overseas record is why. H5N1 killed an estimated 30,000 sea lions in South America within six months in 2022. An estimated 13,359 southern elephant seal pups died on Heard Island, an Australian external territory in the Southern Ocean, between August 2025 and January 2026.
The Pages colony was losing pups before any of this. Pup mortality there reached 34 per cent in the 2025 breeding season against a long-run average of about 5 per cent. State and federal governments put $1.4 million into worming pups for hookworm and building shelters at colony sites in response to those deaths.
Seal Bay Conservation Park remains open and guided tours still run from the boardwalk, but beach access has been paused. Climate, Environment and Water Minister Emily Bourke said the government would look further at a vaccination program of the kind used on seals in Hawaii, and that the regulatory approval such a program would need has not been resolved.
Australia was the last continent the virus reached, in June, which gave governments here years of warning that other countries did not get. The preparation went into surveillance rather than vaccine. New Zealand's Department of Conservation has been running an H5N1 vaccine trial across six species for two years. There is no approved vaccine for a wild Australian marine mammal, and whether one can be used here is a question still with regulators.
In Tasmania, 28 dead little penguins and three crested terns were found at the Port of Grassy on King Island on Saturday. Seven of the eleven bird deaths recorded in the state have been on King Island. Natural Resources and Environment Tasmania incident controller Wes Ford said samples had been collected and sent for laboratory testing, with results expected during the week.
Results on the Pages terns and the King Island birds are both due this week. Sick or dead wild birds should not be touched or approached. Multiple sick or dead birds can be reported to the Emergency Animal Disease Hotline on 1800 675 888, which operates 24 hours.
