Three shark attacks have been reported in Sydney in a little over 24 hours, leaving at least two people seriously injured, according to local authorities.

A man was taken to hospital in critical condition after being injured at Manly beach on Monday evening, New South Wales Police said in a statement.

Hours earlier, an 11-year-old surfer had to be assisted off Sydney’s Dee Why Beach after a suspected shark bit his surfboard, with the local council urging swimmers and surfers to be cautious.

The first incident saw a 12-year-old boy taken to hospital with critical injuries after he was attacked by a large shark while swimming in Sydney Harbour on Sunday afternoon, police said.

The boy and his group of friends were jumping off a six-metre rock ledge at Shark Beach, in the Sydney suburb of Vaucluse, when the attack happened on Sunday.

Despite its name, shark attacks are rare at Shark Beach, a popular swimming spot.

The boy was pulled out of the water by his friends before rescuers arrived. Police said it was their actions that had given him a shot at survival, local media reported. He is currently in intensive care.

“The actions of his mates… have been nothing but brave. [It was] a gallant recovery,” said Joseph McNulty, who leads the state of New South Wales’ marine area command.

“Very confronting injuries for those boys to see, but I suppose that’s mateship.”

Marine police arrived to a “horrendous scene”, McNulty told reporters. They pulled the boy onto a police speedboat and used tourniquets to try and stop the bleeding from his legs.

They also tried to resuscitate the boy as the boat made its way to an ambulance waiting by a wharf.

Authorities believe the boy was attacked by a bull shark, an aggressive species found in warm, shallow waters. Many experts consider bull sharks among the most dangerous sharks in the world.

McNulty believes heavy rainfall over the weekend and brackish water conditions could have created a “perfect storm environment” for Sunday’s attack. Rain flushes nutrients into the rivers and oceans which can draw sharks closer to shore.

The final attack, in Sydney’s Northern Beaches, prompted police to close all beaches in the area until further notice.

According to police, emergency services were alerted to the third attack after a surfer was pulled from the water at North Steyne Beach, Manly, at 18:20 local time (07:20 GMT).

He had suffered serious injuries to his legs, and was rushed to hospital.

Australia, which is home to some of the world’s best beaches, is also one of the deadliest places for shark attacks. There were at least five fatal shark attacks in the country last year.

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