The family of a 12-year-old boy attacked by a shark in Sydney Harbour are dealing with the “worst possible outcome” for their son.

Nico Antic, who was weeks away from turning 13, was attacked after jumping off a six-metre rock with two mates in Vaucluse about 4.20pm on Sunday.

He was rushed to Sydney Children’s Hospital at Randwick with severe leg injuries and was in a critical condition.

A family friend set up a GoFundMe page to support the boy’s family during what was described as an “incredibly difficult time”.

Nico Antic was with friends at Vaucluse when he was attacked.Nico Antic was with friends at Vaucluse when he was attacked. Credit: Supplied

“Tragically, their beloved child, Nico, was attacked by a shark on Sunday in Sydney Harbour and sustained devastating injuries,” the friend wrote.

“Despite all efforts, this heartbreaking event has led to the worst possible outcome.

“Any contribution, no matter the size, would be deeply appreciated and will help ease the financial burden as they grieve.”

The family friend told 7NEWS.com.au Nico was a “very sporty” kid who was “lovely, always happy, loved his relatives and was great at telling jokes”.

NSW Police Superintendent Joseph McNulty said investigators believed a bull shark was to blame for biting Nico’s lower legs.

Nico was treated and taken to hospital.Nico was treated and taken to hospital. Credit: 7NEWS

He praised one of Nico’s friends for jumping into the water to pull him back onto the rock.

“That mate was extremely brave, going into brackish water with the unknown factor of a shark still hanging around,” McNulty said.

There have been four shark attacks along the NSW coastline since Sunday.

Just hours after Nico was attacked, an 11-year-old narrowly escaped injury when a shark took a large chunk out of his surfboard at Dee Why Point on Monday morning.

The 11-year-old boy’s surfboard shows bite marks from a shark. The 11-year-old boy’s surfboard shows bite marks from a shark. Credit: Supplied

Later the same day, a 27-year-old surfer was dragged underwater during a shark attack at Manly Beach just after 6pm.

He was pulled unconscious from the water by fellow surfers and rushed to Royal North Shore Hospital in a critical condition.

Less than 24 hours later, a 39-year-old surfer was reportedly bitten by an “animal” while in the water at Point Plomer Beach, south of Crescent Head, on Tuesday.

He managed to drive himself to Kempsey District Hospital and has since been discharged.

All beaches in Sydney’s northern beaches region have now been closed following the attacks.