Sydney, Australia - A shark mauled a surfer off an ocean beach in Sydney on Monday in the Australian city's third shark attack in two days, authorities said.

The surfer, believed to be in his 20s, was in a critical condition in hospital with serious leg injuries after the attack at a northern Sydney beach, police said.

"The man was pulled from the water by members of the public who commenced first aid before the arrival of emergency services," New South Wales state police said in a statement.

All of Sydney's northern beaches were closed until further notice.

The attack at North Steyne Beach in the suburb of Manly came hours after a shark bit a large chunk out of a young surfer's board about 2.5 miles north along the coast at Dee Why Point.

That surfer, reportedly a boy aged about 11, was uninjured but the beach was closed immediately.

On Sunday, a large shark bit a 12-year-old boy on the legs as he played with friends at a beach in Sydney harbor, leaving him fighting for his life in a hospital.

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 influencing sharks' migratory patterns may be contributing to a rise in attacks despite overfishing depleting some species, scientists say.