Young surfer in Sydney shark attack

An Australian teenager will undergo emergency surgery after being attacked by a shark on Sydney's northern beaches this morning.

The 15-year-old boy was surfing with his father at the northern end of Avalon Beach when the shark attacked just before 9am (NZT), a NSW ambulance spokesman said.

The spokesman said the teenager had been airlifted to Royal North Shore Hospital, where he will undergo surgery after suffering thigh injuries.

"He has severe injuries, but I don't think at this stage they're life-threatening," he said.

"He's got extensive bite wounds and lacerations.

He said he believed the young man's leg had not been severed.

"He was stabilised and treated at the scene and given pain relief.

"Before he left, he was stabilised further so be could treated and flown by helicopter.

"All I know at this stage is that he was surfing with his father."

Nick Miller from the Avalon Beach Surf Lifesaving Club said the surfer was in the water with his father when the shark attacked in the early morning.

"It got him around the top of his leg and his calf muscle and the father came in and dragged him in," he said.

The father and son are members of the surf club.

Network Seven reported that the father rescued his son.

Onlookers wrapped the teenager's wounds in beach towels before emergency services arrived.

A teenage boy told Network Seven he watched the aftermath, with ambulance officers treating the boy on the beach.

"They were just tending to them and we were just watching from up here thinking what's happened," he said.

"And then, on the way here, a surfer said there had been a shark attack."

The father said the attack happened quickly and his son was unable to identify the size or species of shark, Seven said.

No one else was in the water at the beach at the time.

But a short time later, life savers had to call a swimmer out of the surf.

The beach remains empty, with swimmers and surfers warned not to enter the water.

It is the third serious shark attack in Sydney in less than three weeks and comes after repeated warnings by authorities for beachgoers not to enter the water at dawn or dusk due to increased shark activity.

Navy clearance diver Paul de Gelder was lucky to survive after being mauled by a 2.7-metre bull shark off Garden Island, in Sydney Harbour, on February 11, causing him to lose a hand and leg.

Just a day later, 33-year-old surfer Glenn Orgias was attacked by a 2.5m great white that shook him and nearly severed his left hand.

The Sydney Harbour Swim Classic was scheduled to go ahead on Sunday morning despite the earlier attacks.

- AAP