Those in the water were informed of the sighting, and some swimmers chose to leave immediately. By 9am, however, officials erected signs closing both beaches, and few brave souls were still in the water.

One woman who exited the waves at Shelly Beach a few minutes after being told of the sighting declared she was only leaving because of leaky goggles. Her group opted to swim laps near the shore rather than take on the full stretch to Manly and back.

"Because there's a shark, we'll just swim back and forth," she said. "The thing with here is there're always sharks sighted anyway."

The Saturday beach closure comes after Manly resident Peter Schultz was injured on Tuesday during an encounter with what's since been identified as a critically-endangered grey nurse shark.

The experienced ocean swimmer was training before dawn in "reasonably rough conditions" with three others when he was bitten in what fellow swimmer and Surf Life Saving chief superintendent Ken Murphy said was an accidental "collision". Mr Schultz received lacerations to his hand, torso and right thigh.

Mr Schultz, who has previously swum the English Channel, was able to clamber on to nearby rocks. The alarm was raised by his fellow swimmers, including Mr Murphy, while others on shore ran for help or administered first aid.

The bite victim is now recovering at home after a two-day stint in hospital following surgery. Mr Murphy said his friend is doing well, and he expected it would only be about two weeks before he'll be able to return to the waves.

Mr Schultz does not consider his encounter with the "labrador of the sea" as an "attack", Mr Murphy said, referring to the usually harmless shark species.

"I think Pete's like the rest of us - it hasn't been an attack, it's an incident," he said.

"We've been stirring him up a bit about nautical rules and giving way, and things like that. They've crashed into each other, and the shark's had a reaction to that."

"We swim out here regularly during the dark and you do bump into things," Mr Murphy said. "Sometimes there's debris in the water and you think, 'what was that?'"

While the number of swimmers was a little down during the week after Tuesday's incident, Mr Murphy said it was as much to do with the rough surf as it was the shark sightings.

"You've got a group here who are very passionate about their open-water swimming and what we do, so we'll be back in the water," Mr Murphy said. "As a collective, we'll be supportive of each other and look after each other."