A 12-year-old boy is fighting for his life in hospital after a shark attack in Sydney Harbour, police said Monday.

The incident occurred Sunday afternoon as the boy and friends jumped into the water off Shark Beach.

"It was a horrendous scene at the time when police attended. We believe it was something like a bull shark that attacked the lower limbs of that boy," said Superintendent Joseph McNulty, New South Wales marine area police commander.

"That boy is fighting for his life now."

The children were leaping from a six-metre (20-foot) rock in the eastern Sydney suburb of Vaucluse when the attack occurred. Recent heavy rain had turned the harbour water murky.

"We believe the combination of the brackish water, the fresh water, the actions of the splashing may have made that perfect storm environment for that shark attack yesterday," McNulty told reporters.

He praised the boy’s "gallant" friends for rushing to his aid before police arrived.

"He had been bitten by a large shark," McNulty said.

Police pulled the unconscious boy onto a boat and applied two tourniquets to his legs while trying to resuscitate him en route to a wharf, where paramedics took over. He remains in intensive care at Sydney Children’s Hospital, surrounded by family and friends.

Australia has recorded more than 1,280 shark incidents since 1791, with over 250 fatal, according to a national database. Scientists warn that crowded waters and rising ocean temperatures may be shifting shark migratory patterns, contributing to more attacks despite overfishing.

A great white shark killed surfer Mercury Psillakis at a northern Sydney beach in September, and two months later a bull shark killed a woman off a remote northern beach.