The first positive case of H5 bird flu in a mammal was confirmed on Sunday in a long-nosed fur seal at Beachport, 300km south of Adelaide, which authorities believed was contracted from wild birds.
A second seal at The Pages Conservation Park was one of five suspected H5 bird flu events recorded by South Australia's Department of Primary Industries and Regions.
The other suspected bird flu events recorded on Monday involved greater crested terns and a white-fronted tern.
Those birds and the seal have been tested for bird flu, with results expected in the coming days.
The Pages - which covers two small islands in between Kangaroo Island and the mainland - was the site of a mass mortality event of birds, likely due to the H5 strain, last week.
It is a crucial breeding ground for the endangered Australian sea lion, which has a population of less than 12,000.
"If the virus was to reach that species we could see a huge mortality event that could threaten the species with extinction," Antarctic ecologist Jane Younger told AAP.
Australia should expect a "mass die-off" of marine mammals due to the spread, similar to those seen in South America and Heard Island, Curtin University associate professor Bill Bateman told AAP.
"This is a very worrying development," he said.
Before the new case was announced, Environment Minister Murray Watt said the spread from birds to mammals was inevitable and the government was examining whether further vaccination programs were needed.
"We are likely to see mass mortalities of birds and mammals, and I know that is a very distressing thing for Australians to hear. But we also need to be realistic," he told ABC TV on Monday.
H5N1 killed more than 17,000 southern elephant seal pups in Argentina in 2023, and last year about three quarters of the pup population was wiped out at Heard Island, a sub-Antarctic Australian territory in the Southern Indian Ocean.
There is no bird flu vaccine for marine mammals on the market, but a vaccine trial has been undertaken in North America, which Agriculture Minister Julie Collins said the federal government was investigating.
"There certainly is nothing commercially available, but we do continue to look at that research," Ms Collins said.
The last continent to record this strain of the virus, Australia has had "years of notice" that H5 could arrive, Dr Younger said.
"There was time for us to get ready from a vaccine point of view but unfortunately that hasn't happened," she said.
Bird flu vaccinations have been rolled out to at-risk species, including the little penguins on Victoria's Phillip Island.
Many were corralled into pens to receive the first of two vaccinations on Sunday night.
A further 600 penguins vaccinated over the weekend takes the total number having received a first dose at Phillip Island and St Kilda to about 1000.
With case numbers on the rise, authorities have stepped in to do what they can to defend the 5000-strong colony.
The penguin's nightly ritual of returning the same burrow, along the same path, made them ideal candidates for vaccination, said Phillip Island Nature Parks reserve manager Ben Thomas said.
Being a world-first in terms of scale, the process has taken some experimentation and fine tuning.
"We've had our teams out late into the night for a week now and learning a lot along the process," Mr Thomas said.
"We started on some of the quieter penguin pathways to fine-tune the whole process and and get it working as quickly as we can, so we can process as many penguins as we can each night.
"We've now moved to the western end of the penguin parade, where the larger numbers are, and it's working really well."
