A surfer has been airlifted to Adelaide in a critical condition after being attacked by a shark at Fishery Bay, about 40 kilometres south of Port Lincoln.

Police said the 26-year-old man was surfing about 350 metres offshore from Port Lincoln National Park when he was bitten shortly before 10:00am, sustaining serious leg injuries.

He was taken to Port Lincoln Hospital before the MedSTAR helicopter transferred him to Royal Adelaide Hospital.

The Ambulance Service said he needed to be resuscitated.

Police in conjunction with Fisheries SA, local council and Surf Life Saving SA have erected signs on the beach to warn members of the public about the shark attack.

Mayor of the District Council of Lower Eyre Peninsula, Julie Low, said it was a sad incident that will be felt by many locals.

"It's a very popular surfing spot and swimming spot for the local Port Lincoln people," she said.

"It's a beautiful place and it's such a shame this has happened.

"Our sympathy goes out to the victim and his family and friends."

A surf warning sign in Fishery Bay. ( ABC News )

Wildcatch Fisheries SA chairman Jonas Woolford said the number of shark sightings in the area had risen recently.

"There has been a number of sightings just recently actually at a number of locations along the coast near to Fishery Bay, just around the corner, the sightings have said that the sharks have been rather aggressive actually," he said.

"It's been reported that there hasn't been many for about 10 or so weeks, so whether those ones that have been laying low are now coming back to the areas which they normally frequent and they may be rather hungry ... but yes, there has been more sightings around just recently."

He said great white sharks had been scared off by killer whales in the area earlier this year.

"It's only a theory that they may be rather hungry, that they all probably all got fairly freaked out by the killer whales that have been laying very low, so now they might be surfacing again," he said.

"I mean humans in the water, we're very slow and [maybe] they're coming up taking interest and unfortunately, we are easy prey."

A Port Lincoln man reportedly survived a shark attack at the same beach 10 years ago.

The man was also surfing at the beach when he was attacked by a four-metre shark on Father's Day.

He was able to fight off the shark.