A rare sunfish that can weigh more than a car when fully grown washed up at the Coorong National Park in South Australia.

Two fishermen discovered the giant sunfish on the shore over the weekend. Linette Grzelak posted pictures of their find and told The Guardian Australia that her partner thought the dead fish “was a piece of shipwreck at first.”

Ralph Foster, South Australia Museum’s fish collection manager, said this particular type of sunfish was rare.

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"I've actually had a good look at it, we get three species here and this is actually the rarest one in South Australian waters," Foster told the Australian Broadcasting Company.

"It's the oceanic sunfish, which in other parts of the world is common but here it's more unusual, it's the one known as 'Mola mola,'” he continued.

Foster said the washed-up sunfish was medium-sized and that the particular species “can get nearly twice as big as that.”

He said sunfish aren’t fished for because they can do damage to yachts.

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"They are not generally fished for except in parts of Asia," he said.

Foster said photographs of the fish are useful so scientists can learn more about the new species.

"We get to actually look at them so infrequently, so we never know quite which one we've got," he said.