NSW Police say the man is in his 20s. (ABC News)

A man is in a critical condition after another shark attack in Sydney.

Emergency services were called to North Steyne Beach in Manly, on the northern beaches, just after 6pm on Monday after reports a surfer had been bitten by a shark.

Surf Life Saving NSW president Steve Pearce told the ABC the incident was “bad”.

NSW Police said in a statement the man was believed to be in his 20s and was treated for serious leg injuries.

“The man was pulled from the water by members of the public who commenced first aid before the arrival of emergency services,” the statement said.

He was taken to Royal North Shore Hospital.

Multiple medical resources were deployed to the scene to treat him, including intensive care paramedics.

The Toll rescue helicopter was also on stand-by.

All northern beaches have been closed until further notice.

It is the third shark attack in Sydney in just over 24 hours.

Two young boys involved in separate shark attacks

It comes after a 12-year-old boy was attacked at a harbour beach at Vaucluse on Sunday, where he suffered critical injuries to both his legs.

He remains in intensive care at Sydney Children’s Hospital in Randwick.

The Department of Primary Industries and Regional Development (DPIRD) said in a statement they believed that a bull shark was involved.

“DPIRD shark scientists are unable to definitively confirm the species based on photos of the injury,” it said.

“However they believe a bull shark was likely responsible based on the nature of the injuries and the environmental conditions at the site.

“Our thoughts are with the boy, his family and the other young people who witnessed the incident.”

Another boy escaped injury on Monday when his surfboard was bitten at Dee Why on the northern beaches just before lunchtime.

NSW Police were told by witnesses at the scene that the boy involved was an 11-year-old.

Northern Beaches Council said it had activated its “shark sighting protocols” following the incident.

“Council lifeguards immediately sounded the alarm, erected shark warning signage and dispensed a jet ski to patrol the waters,” they said in a statement.

“NSW Surf Life Saving launched a drone and Department of Primary Industries were notified.

“Council lifeguards will continue to patrol for the remainder of the day and tomorrow, and the beach will remain closed for at least 24 hours.”

Resources from police marine area command have been deployed to the beach.

NSW Ambulance said it was not called to the scene.