"The injuries are consistent with what is believed to be a large shark," police said in a statement.
Police pulled the boy, believed to be in his early teens, from the water just minutes after they were alerted to the incident. Aboard a police boat, he received first aid for serious leg injuries before being taken to hospital.
The public is now advised not to swim in the waters near Shark Beach, where the accident occurred.
In September, a great white shark killed a surfer at a popular beach north of Sydney. Two months later, a bull shark killed a woman swimming at a remote beach north of Sydney.
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