She told The Age the fleas were the most likely culprit for Sam's wounds. She added that the number of bites was unusual but that sea fleas are common both in Australia and around the world.

"They're there all the time; you could put a piece of meat in the water, anywhere in the bay, and you could find them," Dr Walker-Smith said.

"I think this is quite a rare thing. I really just think [Sam] was in the wrong place at the wrong time, probably."

Dr Walker-Smith said the fleas were no cause of alarm but that swimmers should avoid swimming near dead fish which the fleas feed on.

A Victorian Department of Environment, Land, Water and Planning spokesman said sea fleas were a "common and natural part of a healthy marine ecosystem" that "keep our marine waters clean by consuming dead and dying marine animals".

"DELWP advises swimmers to wear a wetsuit with boots to reduce the risk of being exposed to sea fleas and avoid swimming at night."