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South Australian police are searching for the body of a man thought to have been killed in a shark attack.

The 55-year-old was surfing near Granites Beach south of Streaky Bay on the Eyre Peninsula on Tuesday morning local time when witnesses reported seeing him being attacked.

The authorities have been scouring the area by air and water with the assistance of local volunteers.

It is the third serious shark attack in the state this year.

Another surfer was killed in May about 120km from Streaky Bay, while a third suffered a non-fatal attack earlier this month.

It is not clear what kind of shark was involved in the attack but witnesses have told local media that it was more than 4m (13ft 1in) in length.

Phil McEvoy, who lives in Streaky Bay, told the Australian Broadcasting Corporation (ABC) that he had heard sirens sounding in the morning "for quite some time".

"I knew then there must have been something wrong, and the sirens sort of went away towards the back beach area where the surf is," he said.

According to 9News, the man was one of six surfers who were in the water at the time but he had paddled out further than the rest.

Mr McEvoy added that the town was currently seeing an influx of surfers and thought that the fishing season might have something to do with a possible increase in sharks in the area.

The waters of South Australia's west coast are well known for the presence of sharks and scientists have cautioned that an increase in sightings of sharks doesn't necessarily mean there is an increase in numbers.

Granites Beach is about 700km (434 miles) from Adelaide, the state's capital.