Gentoo penguin have become the first birds in an Australian territory to test positive for the H5N1 strain of bird flu.
Scientists confirmed the cases on Heard Island after new field samples.
The virus had already been detected there in Southern elephant seal in November 2025.
Further testing has now found infections in fur seals and penguins.
Experts say the jump to bird species is a worrying escalation.
Heard Island lies about 4,000km south-west of Perth in the sub-Antarctic.
The outbreak forms part of a wider spread that has killed millions of birds worldwide.
The disease reached Antarctica in the 2023-24 season and has moved across nearby islands.
Conservationists warn the virus threatens unique local species.
These include the Heard Island cormorant and the Heard Island sheathbill.
Both exist nowhere else on Earth.
The government says mainland Australia remains free of H5N1.
It has invested more than $100m in preparedness and biosecurity.
Researchers stress that long-distance spread makes continued surveillance essential.
