Twenty Sydney beaches have been closed indefinitely as a man fights for his life following a third shark attack in just over 24 hours.

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, at about 6.20pm on Monday, according to NSW Police.

He was pulled from the water by other beachgoers and 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 at Vaucluse on Sunday afternoon.

All of Sydney’s northern beaches have been closed in response until further notice.

A NSW Ambulance spokesperson said the man was bitten on the lower right leg and suffered a “severe” and “life-changing” injury. He had emergency surgery on Monday night.

The man was in cardiac arrest when paramedics arrived on scene and had lost a lot of blood.

He received 13 units of blood while being rushed to hospital, with police meeting paramedics en route to deliver extra blood supplies.

“This is something that doesn’t happen a lot and that is going to be something that is going to give him the best chance of surviving,” an ambulance spokeswoman said on Tuesday morning.

“We also had one of the off-duty NSW Ambulance doctors who happened to be at the beach swimming at the time as well, and he was also a part of treating this patient.

“Everything that the people have done together and the agencies that have worked together is the best chance of survival for this patient.”

Surf Life Saving NSW chief executive Steven Pearce said lifesavers were at a nearby surf club doing CPR and first aid training and were able to race to the scene.

“They were able to immediately leave the building, get on to the beach, take the defibrillators, take the shark tourniquet kits and get there and apply a tourniquet and apply CPR,” he said.

“There were two other off-duty doctors also on scene to assist with the CPR effort and, no doubt, that quick and timely immediate first aid response has really given this gentleman a great chance of survival.”

Earlier on Monday, an 11-year-old was surfing at Dee Why Beach when a shark bit his board several times, taking a chunk out of the mid-section.

The 15-centimetre bite mark on the young surfer’s board suggests he was most likely attacked by a bull shark, according to biologists from NSW’s 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.

An 11-year-old boy has escaped a shark at a Sydney beach, less than a day after another boy suffered critical injuries.

The young surfer was unharmed and helped out of the water by others, including his father.

Northern Beaches Council lifeguards immediately sounded the alarm, put up shark warning signs and dispatched a jet ski to patrol the water.

Surf Life Saving NSW launched a drone to assist but there have been no further sightings since Monday morning’s attack.

“Council lifeguards will continue to patrol for the remainder of the day and tomorrow,” a Northern Beaches Council spokesperson said.

That attack came 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.

Mercury Psillakis died after a shark attack in September.

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 said before the Dee Why incident was reported.

Heavy swell has also prevented 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 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, 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.”

NSW authorities are 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.

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.

-with AAP