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 4 kilometers (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, AFP reported.
On Sunday, a large shark bit a 12-year-old boy in the legs as he played with friends at a beach in Sydney harbor, leaving him fighting for survival in hospital.
The boy and his friends were jumping from a six-meter (20-foot) rock into the water off Shark Beach in the eastern suburb of Vaucluse when the predator struck, police said.
"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," he told reporters on Monday.
Recent heavy rain had drained into the harbor, and authorities believed the combination of the brackish seawater and the children's splashing created a "perfect storm" for a shark attack, McNulty said.
He warned people not to go swimming in the harbor or other river systems in New South Wales because of the risks.
He praised the boy's "brave" young friends for pulling him out of the water on Sunday.
Officers put the unconscious child in a police boat and gave him first aid, applying two tourniquets to stem the bleeding from his legs, McNulty said.
They tried to resuscitate the boy as they sped across the harbor to a wharf where ambulance paramedics were waiting.
The child, confirmed by police to be 12 years old, was in intensive care at Sydney Children's Hospital surrounded by family and friends, McNulty said.
