The latest case was detected in a giant petrel found unwell at Knights Beach in Port Elliot, about 80km from Adelaide on June 14, Agriculture Minister Julie Collins confirmed on Wednesday.
It is the first definite case in the state after two earlier confirmed results in birds found on a remote beach in Esperance, about 700km southeast of Perth.
Urgent testing is under way after another suspected case was detected in a giant petrel seabird found in Quindalup, southwest of Perth, on Sunday.
Authorities confirmed on Wednesday that samples had been sent to specialist CSIRO laboratories in Geelong for further analysis, which would be provided in coming days.
Samples from a dead porpoise found in the Esperance region over the weekend have been sent for testing, although it's not known when it died.
"This is obviously concerning," Ms Collins said.
"But given the spread of (the virus) globally, it is not unexpected that other migratory birds may have arrived at other locations across the Australian coastline."
A temporary ban on all poultry products imported from Australia, imposed by Papua New Guinea's National Agriculture and Quarantine Inspection Authority, has been lifted after being put in place on Monday.
"We appreciate PNG's willingness to work co-operatively and to remove some of these restrictions," Ms Collins said.
PNG is Australia's largest single market for chicken meat exports, accounting for nearly half of the $133 million in sales in 2023/24.
Following the detection in South Australia, the state's premier confirmed there were no cases in local poultry and said authorities were treating the discovery as an isolated incident.
"I know this news will be concerning for many people, particularly our poultry producers, wildlife carers and communities across our state," Peter Malinauskas said.
"I want to reassure that we are well prepared and responding swiftly."
WA Agricultural Minister Jackie Jarvis said there had been no evidence of mass mortalities among birds following the initial discovery of the sick animals.
Authorities have received nearly 200 reports of sick birds since Friday's first suspected case, including 79 in the past 24 hours.
"Testing of samples collected is ongoing and results will be provided when available," Ms Jarvis said on Wednesday.
The poultry and agricultural sectors in Australia remain free from the flu, although the industry has been placed on high alert since the cases were confirmed in wild birds.
Major poultry producer Inghams is locking down its WA sites to mitigate against potential risks.
More than 200 million chickens have been culled in the US since the virus arrived there.
Australia had been the last continent free of the virus strain, although Ms Jarvis said it was not uncommon for seabirds to arrive on shore sick or exhausted.
"Winter is a time when we see migratory seabirds come ashore, (they are) birds that would not normally land in WA, and so we ask the public to remain alert," she said.
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