a man is in a critical condition after a shark attack in south australia .
police said the man was attacked 350m offshore at fishery bay .
the man was rushed to hospital with life-threatening injuries .
the attack happened shortly before 10 am on saturday .
witnesses claimed a great white shark has bitten off the man 's leg .
the shark victim has been airlifted to adelaide for further treatment .
south australia has been the site of numerous shark attacks .