The Westpac Lifesaver Rescue Helicopter was patrolling over Marino Rocks, in Adelaide's south, on Sunday afternoon when crew members spotted the shark about 200 metres off shore.

A Surf Life Saving South Australia spokeswoman said the helicopter's crew members estimated the shark to have been six to seven metres long.

"They didn't have a tape measure, of course, but that is how long they estimated it to be," the spokeswoman said, adding that crew members regularly saw sharks from the air, and that this was a particularly large one.

She was unable to comment on an anonymous post on the Shark Alerts South Australia Facebook page, said to have been written by one of the crew members.

The post read: "It's the biggest I've ever seen as a crew member in the Westpac chopper. We put it down as 7m the photo really doesn't do it justice."

The spokeswoman said the junior surf lifesaving carnival was being held at neighbouring Seacliff Beach at the time.

"We had our jet boats offering water coverage at the carnival, and they were called into action, as were the inflatable rescue boats," the spokeswoman said.

"By the time they got there, the shark had moved into deeper waters. There wasn't another sighting of the shark ... and it was moving fairly quickly into deeper water."

The surf life saving carnival was called off early, and swimmers were evacuated from the water.

smh.com.au