A sunfish has been found washed ashore near the mouth of the Murray River in South Australia over the weekend, and one expert believes it is the Mola mola species.

Key points: The sunfish was spotted by a group of fishermen along the Coorong on Saturday

The sunfish was spotted by a group of fishermen along the Coorong on Saturday An expert says this particular species is rare for South Australian waters

An expert says this particular species is rare for South Australian waters He said sunfish had been known to "sink yachts" due their sheer size

The gigantic fish is known for its sheer size and odd body shape, often distinguished by its flattened body and fins.

A photo of the creature was posted on social media on Monday, with two fishermen standing over it on the sand.

Linette Grzelak, who posted the image to Facebook, said the fish — which was already dead — was spotted by her partner while out fishing on Saturday.

"A sunfish found by my partner along the Coorong a couple [of] nights ago … I thought it was fake," the post said.

Sunfish can swim to the depths of the ocean

South Australian Museum fish collection manager Ralph Foster said this particular species was a rare find.

"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," he said.

"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."

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He said this particular sunfish was on the smaller end of the scale in terms of size.

He estimated it was about 1.8 metres long.

"They can get a lot bigger … it's probably an average-sized one, they can get nearly twice as big as that," he said.

Mr Foster said the sunfish got its name for its trait of basking in the sun, but they were also known to swim to the depths of the ocean.

"In recent work people have been putting satellite tags and data loggers on them and found they will come to the surface and lay on their side on the surface, hence the name the sunfish," he said.

"Once they are warm enough they will dive down several hundreds of metres and feed on jellyfish and things right at the depth and stay down there for quite a long time.

"We know very little about them, it's only in the last few years that technology has allowed us to start learning about them."

Sunfish are known to 'sink yachts'

Mr Foster said the sunfish was not commonly fished for and had been known to have damaged yachts.

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

"We get a lot of them hit by boats and some of them are so large they actually sink yachts.

"I think one female can lay something like 3 million eggs, so they are not particularly endangered."

Marine scientist Marianne Nyegaard with a beached hoodwinker sunfish found earlier this year. ( Supplied: Murdoch University )

He said various marks on its tail and the shape of its head made him confident the latest find in South Australia was a Mola mola sunfish, but there was a lot more to be discovered about them.

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

"Which is why these photographs online are so useful, because we get to actually look at it and decide which one it is.

"But they are amazing things, they really are."

The find comes as a rare hoodwinker sunfish was recently found washed ashore in California earlier this year.

The 2.1-metre fish was spotted at the Coal Oil Point Natural Reserve and initially left scientists baffled when they could not identify it.

Marine scientist Marianne Nyegaard, from Murdoch University, identified the new species after analysing more than 150 samples of sunfish DNA.