The sunfish was estimated to be around 2.5m in length and weighed several hundred kilograms.

Linette Grzelak posted the image of the fish to Facebook after her partner, who was fishing for cockles, returned home.

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

‘They thought it was wood as they were driving up,’ she added.

Sunfish can grow up to 3m long, 4.2m high and weigh up to 2.5 tonnes are considered vulnerable in the wild making the find even more incredible.

They are found in tropical waters around the world and are often confused for sharks due to their fin.

The large fish is considered a delicacy in some parts of Asia including Japan, Korea and Taiwan.

Fish collection manager Ralph Foster, from the South Australian Museum, explained why so many sunfish get washed up on the beach.

‘One of the big dangers would being hit by big boats at sea,’ he said.

‘They often eat plastic bags thinking they are jelly fish which can kill them.’

Sunfish wash up on South Australian shores quite frequently, Mr Foster said he received reports of several every year.

‘They are actually quite common in Australian waters but they are generally further out to sea.’