Three shark attacks have been reported in Sydney in a little over 24 hours, leaving at least two 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, leading to a significant chance of survival. He is currently in intensive care.

The actions of his mates... have been nothing but brave. [It was] a gallant recovery, said Joseph McNulty, leader 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 as they tried to stop the bleeding from the boy's injuries using tourniquets.

Authorities believe the boy was attacked by a bull shark, an aggressive species commonly found in warm, shallow waters and considered one of the most dangerous sharks globally.

Heavy rainfall over the weekend may have created a perfect storm environment for the attack, as nutrients washed into the water can draw sharks closer to the coast.

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

This alarming spike in shark attacks comes as Australia remains one of the deadliest places for such incidents, with at least five fatal attacks reported last year.