Three cases of the deadly avian influenza have been confirmed so far in seabirds in two states in Australia over the past week.
There are also two suspected cases, the latest discovered among four other birds at Roses Beach near Esperance on June 21.
The petrel is still awaiting further confirmation through testing, WA Agriculture Minister Jackie Jarvis told reporters on Friday.
The other birds found at Roses Beach returned negative results, which Ms Jarvis said was an encouraging was result.
"It means there is no evidence of disease transmission here on the Australian mainland," she said.
"Australia is well prepared to respond quickly to H5 bird flu to protect poultry and reduce impacts on wildlife."
Samples from the bird, a petrel common to the subantarctic, have been sent to the CSIRO for further testing in Geelong.
WA authorities have received more than 357 reports of sick birds since the first suspected case found on a remote beach in Esperance, about 700km southeast of Perth.
About 43 have been prioritised for further investigation and testing, with 12 returning negative results.
Confirmed cases include a brown skua found in Esperance, WA, on June 14, a giant petrel found on the Fleurieu Peninsula, SA, on June 14, and a giant petrel found in Esperance on June 18.
Suspected cases include a giant petrel found in Quindalup, WA, on June 20, and the giant petrel found on June 21.
Ground-based surveillance and drone surveys continue across the country, with poultry and egg producers remaining on high alert.
Major poultry producer Inghams earlier this week announced it would lock down its WA sites to mitigate against any potential risks.
"There have been no detections in poultry, and there have been no reports of large scale mortalities," Ms Jarvis said.
A temporary ban on all poultry products imported from Australia, imposed by Papua New Guinea's National Agriculture and Quarantine Inspection Authority on Monday, has been lifted.
Opposition spokesman Lachlan Hunter urged the state government to continue taking precautions to protect poultry and egg producers.
"I've spoken with industry representatives and the message is clear. Producers want to ensure every reasonable step is being taken to protect Western Australia's poultry and egg industries," Mr Hunter said.
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