Beaches from Manly to Freshwater were closed after a shark tag sensor detected a tagged Tiger shark at the North Steyne receiver.
Swimmers from North Narrabeen to Collaroy were also on high alert after a passing vessel reported sighting a shark in excess of six metres around 200 metres off North Narrabeen.
Northern Beaches Council lifeguards and Surf Life Saving NSW patrols entered the water on jet-skis to search the area and move the shark away from the shore.
Beaches have since re-opened due to no further shark sightings.
Swimmers have been urged to swim at patrolled beaches, exercise caution and exit the water if a shark alarm is sounded.