A surfer is fighting for life after he was attacked by a shark near Port Lincoln in South Australia.

Police say the man, 26, was surfing about 350m offshore at Fishery Bay, in Port Lincoln National Park, when he was attacked shortly before 10am yesterday.

The surfer, who has been named in News Corp newspapers as Chris Blowes, lost his leg from the mid-thigh in the attack.

Fellow surfers risked their lives to help the critically injured man to shore.

An Ambulance South Australia spokeswoman said he was given CPR and a helicopter was dispatched to Port Lincoln hospital.

He remains in a critical condition.

Witnesses said the shark appeared to be a great white.

"The shark came in and bit his leg off, and the guys helped him in and carried him up the cliff," a witness told The Advertiser newspaper.

"I was just watching the shark go out to the ocean with his board still attached.