Nurse sharks are usually bottom-dwellers and do not generally bother humans (Representational)

A swimmer was injured at one of Sydney's popular beaches Tuesday, in what Australian authorities described as an attack by a "generally harmless" shark.

New South Wales police said the man was receiving treatment in hospital for a "puncture wound to his back and injuries to his leg and stomach".

"Around 6am, the man -- believed to be aged in his mid-50s -- thought he had been bitten while swimming with friends at Shelly Beach."

The man was "able to swim to nearby rocks and raise the alarm," police said.

The New South Wales regional government said its scientists had assessed photos of the wounds and "believe a generally harmless grey nurse shark was responsible."

Nurse sharks are usually bottom-dwellers and although large with a mouth full of serrated teeth, do not generally bother humans.

Ocean swimming is a popular pastime in the Australian city, even in winter when temperatures drop into the single digits centigrade.