SYDNEY (AFP)

A shark mauled a teenage boy in Sydney Harbour on Sunday, police said, leaving him in a critical condition with serious leg injuries.

The predator bit the boy, believed to be about 13 years old, in the late afternoon off Shark Beach, New South Wales state police said.

"The injuries are consistent with what is believed to have been a large shark," police said in a statement.

Officers pulled the teen from the water, off the harbour beach in the eastern Sydney suburb of Vaucluse, within minutes of being alerted to the incident, police said.

They gave the boy first aid for "serious" leg injuries while he was aboard a police boat, applying two medical tourniquets.

Paramedics transported him to Sydney Children's Hospital, where he was said to be in a critical condition.

"Swimmers are advised to avoid entering nearby waters at this time," police said.

There have been more than 1,280 shark incidents around Australia since 1791, of which more than 250 resulted in death, according to a database of the predators' encounters with humans.

Increasingly crowded waters and rising ocean temperatures that appear to be swaying sharks' migratory patterns may be contributing to a rise in attacks despite overfishing depleting some species, scientists say.

A great white shark mauled surfer Mercury Psillakis to death at a popular northern Sydney ocean beach in September last year. The man, who left a wife and young daughter, suffered extensive injuries and his surfboard was broken in two.

Two months later, a bull shark killed a woman swimming off a remote beach north of Sydney.