Five suspected H5 bird flu cases were recorded in South Australia on Monday, including a long-nosed fur seal at The Pages Conservation Park, the state's Department of Primary Industries and Regions said in a statement on Monday morning.
The Pages - which covers two small islands in between Kangaroo Island and the mainland - is a haven for the endangered Australian sea lion.
The fur seal has been tested for bird flu, with results expected in the coming days.
The first positive case of the virus in a mammal was confirmed on Sunday in a long-nosed fur seal at Beachport, 300 kilometres south of Adelaide, which authorities believed was contracted from wild birds.
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.
"This is what we've seen happen overseas. Australia is very fortunate that we've been the last continent on Earth to have this strain of bird flu happen. But we are likely to see those kind of events as we have overseas."
Agriculture Minister Julie Collins said the federal government was investigating whether bird flu vaccines were available to be used on mammals.
"There are trials underway in North America, in California and Hawaii, and this is using a vaccine that's been developed for cattle," she told ABC Radio.
"There certainly is nothing commercially available, but we do continue to look at that research."
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.
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."
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.
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