A brown skua found dead in the Rocky Cape National Park on Tasmania's northwest coast was on Thursday confirmed to have tested positive to the H5 strain.
The strain, which arrived on the Australian mainland in June, has now been detected in all jurisdictions bar the Northern Territory and ACT.
There is no evidence of an outbreak in Tasmania or cases in commercial birds. There have been no cases in domestic birds nationwide.
Rachael Alderman, director of threatened species in the state's environment department, said there were genuine worries about how the virus might impact the Tasmanian devil.
The carnivorous marsupial, which has been decimated by a deadly facial cancer, is listed as endangered.
"Devils love to scavenge on dead things. There is every chance that devils would eat infected birds," Dr Alderman said.
"Whether or not the virus would actually infect devils, that is an uncertainty. But that is something we're very concerned about."
The national park where the dead skua was found is home to a range of mammals including Tasmanian devils.
The H5 strain also posed a "significant threat" to the critically endangered migratory orange-bellied parrot, which flies between southwest Tasmania and southern mainland Australia.
The species' wild population of around 90 birds is propped up by a breeding program and the release of captive-bred individuals each year.
There are plans to vaccinate several hundred captive parrots as part of a national bird flu response plan.
It was likely this year's release of captive-bred parrots would still go ahead because their population was dependent on it, Dr Alderman said.
As of Wednesday night, there had been 151 confirmed H5 bird flu detections across Australia, in SA (93), Victoria (43), WA (10), NSW (four) and Queensland (one).
The H5 strain, which arrived in Australia in June, has killed hundreds of millions of poultry worldwide and devastated wild bird and marine mammal populations.
The infected brown skua, a migratory seabird, may have arrived in Tasmania from a subantarctic breeding island, Australasian Seabird Group scientist Eric Woehler said.
Recent passages of cold fronts and associated strong winds are likely to have pushed the infected bird closer to shore, Dr Woehler said.
Increased aerial surveillance of the national park would be undertaken, state Primary Industries and Water Minister Gavin Pearce said.
Mr Pearce urged Tasmanians to remain vigilant and report sick or dead birds and wildlife, while ensuring wild birds don't access backyard chicken food and water sources.
The state had developed a response plan and was working with the commercial poultry sector and to protect at-risk wild species, he said.
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