A long-nosed fur seal has died from H5 bird flu in South Australia, marking the first confirmed case and fatality in a mammal on the Australian mainland.

The animal was found dead on Wednesday at Beachport on the state's Limestone Coast.

The discovery marks a fresh escalation in the spread of the virus, coming just days after South Australian Premier Peter Malinauskas announced three mass mortality events on offshore islands.

Those island events involved the deaths of more than 1,000 greater crested terns, signalling significant local transmission among wild bird populations.

South Australian authorities have also confirmed that the virus has reached other coastal wildlife. A little penguin found on Kangaroo Island tested positive for H5 bird flu this week.

To put this into perspective, this highly pathogenic strain of bird flu has already killed millions of birds globally as it continues to circulate worldwide.

The disease has also repeatedly crossed species barriers outside Australia, infecting and killing various marine mammals, including elephant seals.

The risk to Australia's native wildlife remains a major concern for officials. The federal government had already assessed long-nosed fur seals and the endangered Australian sea lion as facing a very high risk from the virus.

What we do not know yet is whether this mainland seal death represents an isolated crossover event or wider undetected transmission among mammal colonies along the coast.

I will keep you updated as authorities monitor the situation and further test results emerge. I'll be following this one — check back for updates.