The latest confirmed case involves a giant petrel in the state of Western Australia, the country's chief veterinarian, Beth Cookson, said in a statement on Saturday.
Two cases have previously been detected in the same state and another in the state of South Australia.
Samples from another giant petrel have been sent for analysis.
There are no signs of mass mortality, and the virus has not been found in domestic birds or in agricultural production. The risk to human health remains low, Cookson said.
Earlier this summer, Australian researchers reported that a variant of bird flu, the H5 virus, had killed over 13,000 seal pups on a remote and uninhabited island group.
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