Swimmers were rushed out of the water at Bondi when an alarm sounded for a possible shark sighting about 1.15pm on Saturday.

But what fishermen thought was a big shark, turned out to be a large school of fish creating a dark patch in the water.

Westpac Life Saver Rescue Helicopter pilot Jon Clopper said he searched the whole beach for 15 minutes, but no shark was found.

He returned to shore about 2.20pm.

''We just got back and had a look, there was a school of fish,'' he said. ''If there was a shark it would attract them, but we didn't see any.

''The school of fish moved along the shore out of the area.''

North Bondi resident Vanessa Vartto said when an alarm signalled people rushed out of the water.

Ms Vartto said about 25 minutes later a helicopter began circling the beach.