Sydney  - A 15-year-old boy surfing with his father became the third shark attack victim in Sydney in as many weeks when he was mauled at picture-perfect Avalon Beach on Sunday.

"He's got extensive bite wounds and lacerations," a spokesman for Royal North Shore Hospital said. "He has severe injuries but I don't think at this stage they're life-threatening."

Nick Miller from the Avalon Beach Surf Lifesaving Club said the pair were the only surfers in the water when the boy was gored just after dawn.

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

Witness David Rundle praised the young surfer's courage. "When we asked him about the pain, out of one-to-10, he said it's a 10," Rundle said.

Avalon local Alan Low said it was the first shark attack at Avalon but warned of more to come as shark sightings were becoming more common.

"I've been here all my life and we get the occasional shark-spotting, but this year has been really different," he said.

Experts say the water around Sydney is becoming cleaner and warmer and that this has brought more fish closer to shore - and more of the sharks that prey on them.

It's the third shark attack in Australia's biggest city in less than three weeks and follows fresh warnings for people not to swim at dawn or dusk - even to avoid the water altogether.

A navy diver lost a hand and a leg to a 2.7-metre bull shark within sight of the Sydney Harbour Bridge on February 11. The following day, a 33-year-old surfer was lucky not to lose his arm after an attack by a 2.5-metre great white shark at iconic Bondi Beach, the city's most popular strip of sand.

News of the Avalon attack came just before a record field of 880 swimmers jumped into the water at the Opera House for the Sydney Harbour Swim Classic.

Event organizer Adam Wilson said that he was delighted that more people had entered than in any of the previous eight years.

There were shark spotters on the Opera House, underwater patrols and a flotilla of surf lifesavers on surfboards to guard against both sharks and accidental drownings.

"These are the general public swimming in this, so we've got to make sure for their safety, more so than what's in the water, we're looking after the swimmers," he said.

First-time competitor Lindy Woodrow told Deutsche Presse-Agentur dpa while waiting for the gun to go that she was more frightened of runaway shopping trolleys at her local Aldi supermarket.

"There's more chance of being knocked down crossing the road than being attacked by a shark," the 55-year-old said. Asked whether the Avalon shark attack up the coast had made her think twice about the 2-kilometre dash, Woodrow quipped: "No, that means they're all up there and not here." dpa