A giant petrel found at Roses-Quallup Beach at Esperance in southwest Western Australia was confirmed on Tuesday as having the virus.
There's still no signs of spread to local poultry or any evidence of mass mortalities in wildlife, Agriculture Minister Julie Collins said.
Four other dead petrels found near the positive case returned negative tests but Australian Chief Veterinary Officer Beth Cookson said it wasn't possible to rule out whether they had actually been infected.
"The detections so far have been in wild migratory seabirds, and there's no evidence of spread to resident populations," Dr Cookson told reporters in Canberra.
"These birds are active in southern waters during this period of year.
"Our activities will continue to assess the situation over the coming weeks and months, including through enhanced surveillance."
There were no additional suspected positive tests after a sick migratory bird found in Victoria tested negative, Ms Collins said.
Australia had been the only continent free of the highly contagious virus, which has wiped out millions of birds worldwide and also killed mammals.
Four birds have returned positive results for the strain since it was first detected on the Australian mainland on June 14.
They were a brown skua found in Esperance, WA, on June 14, a giant petrel on South Australia's Fleurieu Peninsula on June 14 and a giant petrel in Esperance on June 18.
A positive case in a giant petrel from the WA southern coast was confirmed on Saturday.
The risk to human health remains low, Dr Cookson said.
A temporary ban on poultry products from Australia imposed by Papua New Guinea's National Agriculture and Quarantine Inspection Authority was lifted on June 24.
People are being urged to avoid contact with sick or dead wildlife but report any finds to an emergency hotline and record locations and take photos.
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