The Department of Agriculture, Fisheries and Forestry said on Saturday a silver gull found at Robe in South Australia had been confirmed positive for H5 bird flu.
"This is the first confirmed detection in a silver gull in Australia," Chief Veterinary Officer Beth Cookson said.
Burnet Institute chief health officer Suman Majumdar said the fresh case was significant.
"Gulls move between beaches, wetlands, farmland and urban areas, which could carry the virus into a much wider range of wildlife, poultry and environments," he told AAP.
Associate Professor Majumdar said overseas experience suggested Australia should expect further spread along coastlines, mass mortality events in seabird colonies and marine mammals, and rising detections in the weeks ahead.
"What we are seeing now is the early part of that curve," he said.
Australians have previously been warned to brace for mass deaths in seabird colonies and marine mammals, as the deadly virus spreads.
National leaders met on Friday to discuss Australia's ongoing response to the virus, which now spans five states.
Agriculture Minister Julie Collins said they vowed to do "everything we can" to help protect Australia's wildlife, agricultural industries and the national economy amid the outbreak.
The meeting was briefed by experts on this week's decision to enact the H5 National Management Agreement.
The call was based on the advice of experts that the H5 bird flu in wildlife in Australia was not technically feasible to eradicate, Ms Collins said.
"What we do know is we have that local transmission in the greater crested terns in South Australia," Ms Collins said.
The federal government had invested $113 million to prepare for the arrival of the flu in Australia, and released a further $24 million to states and territories in July.
The total number of presumed positive cases soared late on Friday when 19 greater crested terns from several locations along SA's southeast coast and Kangaroo Island were confirmed for bird flu.
All were wild seabirds found in coastal locations.
There have been no detections in poultry or in Australia's agricultural production system.
The risk to human health remains low.
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