Sea rescue patrols are warning boats off the southern tip of South Australia's Fleurieu Peninsula after a man was bitten by a shark this morning.

Witnesses say the 24-year-old man was part of a group spearfishing at Second Valley, south of Adelaide.

He sustained bites to his hand and ankle, but is conscious and talking to paramedics.

He was treated at the scene before being taken to a local medical centre at Yankalilla.

SA Ambulance says he is now being transferred to the Flinders Medical Centre.

Witnesses report there have been a number of shark sightings in the area during the past three weeks, including one estimated to have measured nearly six metres.

The shark, which attacked today, is thought to have only been about two metres long.