The 51 New Zealand fur seal pups found on a South Australian beach may have abandoned.
Vets remain baffled by the unprecedented deaths despite post-mortem examinations on Tuesday on three of the pup bodies found dead on a beach on the Eyre Peninsula on Sunday.
Adelaide University veterinary pathology lecturer Lucy Woolford said the tests were so far inconclusive but further samples would be examined.
"They weren't emaciated, there were no obvious signs of trauma," Dr Woolford told AAP.
"We are thinking it's either an infectious cause or some kind of incident that has caused an abandonment-like situation or mismothering."
The bodies of an adult and a sub-adult seal were also found with the pups, taking the death toll at the beach to 53.
Dr Woolford said there were up to 2000 NZ fur seals in the area.
Mother seals foraged for days at a time to find food for pups, she said.
The pups appear to have all died at a similar time and been washed ashore.
But the badly decomposed remains were at least five to 10 days old, making it difficult to obtain a conclusive result, she said.
"Something caused these pups to be separated from their mother," Dr Woolford said.
Dr Woolford said the deaths were "sad and unprecedented" and urged the public to inform authorities of any more seal deaths.
She said the university staff were also investigating whether severe weather may have contributed to the deaths.
The Department of Environment and Natural Resources said the pups washed up along six kilometres of coastline in the Wanna Beach area of Lincoln National Park.
New Zealand fur seals are found along Australia's south coast and along the coast of New Zealand's South Island.
A protected species, they can weigh up to 250kg but males usually average about 125kg.
They are generally considered docile but will attack if provoked.