The man, believed to be in his 20s, is in a critical condition after he was bitten by a shark at North Steyne Beach at Manly in Sydney's north about 6.20pm on Monday evening, according to NSW Police.
He was pulled from the water by members of the public and has been taken to Royal North Shore Hospital to be treated for serious leg injuries.
It was the third shark attack at a Sydney beach in two days after an 11-year-old surfer had his board bitten by a shark at Dee Why just a few kilometres away, and a 13-year-old boy was attacked while swimming in Sydney's east on Sunday.
All of Sydney's Northern Beaches have been closed in response until further notice.
The 11-year-old was surfing at Dee Why Beach earlier on Monday when a shark bit his board several times, taking a chunk out of the mid-section.
The surfer was unharmed and helped by others out of the water.
Northern Beaches Council lifeguards immediately sounded the alarm, erected shark warning signs and dispatched a jet ski to patrol the water.
Surf Life Saving NSW launched a drone to assist with the investigation but there have been no further sightings since the attack before midday.
"Council lifeguards will continue to patrol for the remainder of the day and tomorrow," a Northern Beaches Council spokesperson said.
The 15cm bite mark on the young surfer's board suggests he was most likely attacked by a bull shark, according to shark biologists from the primary industries department.
The shark net had been set as normal on Dee Why Beach, which was closed at the time of attack because of large swells and dangerous surf.
The attack comes less than 24 hours after three teenagers heroically leapt into action to save the life of a friend who was critically injured in a shark attack in eastern Sydney.
The 13-year-old boy remains in hospital with injuries to both legs after being bitten by a shark at a swimming spot in Vaucluse on Sunday afternoon.
Dee Why is also a stone's throw from where avid surfer Mercury Psillakis, 57, was fatally mauled by a shark in September.
Police on Monday urged people not to swim in murky, low-visibility water after bucketing rain drenched Sydney and its surrounds at the weekend.
Extra fresh water in the harbour after recent heavy rain, combined with the splashing effect from people jumping off a rock face, created a "perfect storm" for Sunday's shark attack, Superintendent Joseph McNulty said.
"I would recommend not swimming in the harbour or our other river systems across NSW at this time," he told reporters before the Dee Why incident was reported.
Heavy swell has also prevented the operation of smart drumlines, which notify authorities of shark activity along most of the NSW coast since Sunday.
The teenager injured in Sunday's attack underwent surgery overnight and remained in a critical condition at Randwick Children's Hospital on Monday, surrounded by family.
He and his friends were jumping off a six-metre rock into the ocean when the shark struck.
At least one of the victim's friends jumped into the water and pulled him out after the attack, while the others called for emergency services, Supt McNulty said.
"The actions of his mates who have gone into the water to pull him out have been nothing but brave," he said.
"Those actions of those young men are brave under the circumstances and very confronting injuries for those boys to see, but I suppose that's mateship."
The primary industries department is working to identify the species of shark involved but initial reports suggest it was a bull shark.
Multiple swimmers have been attacked by sharks in the harbour in recent years, including a young woman who was bitten on the leg at Elizabeth Bay in early 2024.
Mr Psillakis's death - at Long Reef Beach on Sydney's northern beaches - caused the state government to pull a trial to remove a handful of shark meshing nets off popular beaches.
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