A fourth person has been bitten by a shark in NSW amid warnings as to what could be causing the spate of horror incidents.

A 39-year-old man was attacked on Tuesday morning near a popular campground at Point Plomer, north of Port Macquarie, on the mid-north coast.

It was the fourth attack on NSW’s coastlines since Sunday, leading to a swathe of beaches being closed.

The latest victim was in a stable condition at Kempsey Hospital, a Mid North Coast Local Health District spokeswoman confirmed.

Emergency services had no record of transporting the man to hospital, with reports suggesting a witness on the beach drove him there.

Several beaches in the local area, which includes surfing hotspot Crescent Head, have been closed.

Late on Monday a man was fighting for his life after an attack at a beach in Manly, and on Sunday afternoon a 13-year-old boy was mauled at Vaucluse.

On Monday morning, an 11-year-old boy narrowly avoided injury at Dee Why, with a shark taking a chunk out of his surfboard.

A family friend says Nico Antic has suffered devastating injuries from a shark attack. Photo: GoFundMe

The first boy who was attacked on Sunday afternoon, Nico Antic, had been jumping off rocks with mates at Nielsen Park when a shark attacked his legs.

A man who said he was a family friend launched a GoFundMe appeal on Tuesday, aiming to raise $50,000 to help with the costs of dealing with the avid young soccer player’s “devastating injuries”.

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

“We are raising funds to help the Antic family cover upcoming expenses and related arrangements during this incredibly difficult time.”

Surf Life Saving NSW chief executive Steven Pearce said “turbid and brackish” water from recent weather events had created prime conditions for bull shark activity.

James Cook University marine biologist Jodie Rummer said a combination of very warm water, active weather systems and recent heavy rainfall along the NSW coast had likely drawn sharks to food plumes near rivers and stormwater drains.

“We also have more people in the water than ever before, which naturally increases the chance of encounters,” she said.

The key was being shark smart, including avoiding turbid waters after storms and following local advice and beach closures.

“Sharks are a sign of a functioning ocean, not a broken one,” she said.

University of Sydney public policy expert Chris Pepin-Neff, who has written a book on shark policy, suggested Sydney Harbour’s 100-year-old sewage pipes were likely to blame.

The pipes overflow with just 20 millimetres of rain. More than 120 millimetres fell on Sunday.

“When the pipes overflow, when there’s more sewage, the bait fish eat the sewage and then the bull sharks come to eat the bait fish, and that draws them into the surface and into the shore,” Pepin-Neff said.

“Three shark bites in two days suggests to me there’s a unique environmental condition that is more than just the heavy rain.”

State officials are working to identify the species of shark involved in each incident.

Paramedics earlier revealed a formula one-style pit stop to deliver extra blood might have saved the life of the man who was mauled in Sydney’s north on Monday night.

The man, aged in his 20s, was bitten at Manly’s North Steyne Beach and had surgery for severe lower-leg injuries.

He remains in Royal North Shore Hospital in a critical condition after requiring 13 units of blood at the beach and during the drive to hospital. Such was the extent of the man’s heavy blood loss, highway police met treating paramedics en route to drop extra supplies.

“They said it was almost like an formula one pit stop of 10 seconds or less where they just opened the door, blood went in and then they continued on,” NSW Ambulance acting superintendent Christie Marks said.

“This is something that doesn’t happen a lot … that is going to give him the best chance of surviving.”

On Sunday, a 13-year-old boy was attacked while swimming in Vaucluse in Sydney’s east on Sunday, requiring surgery for severe injuries to both legs.

On Monday morning, an 11-year-old boy narrowly avoided injury at Dee Why, with a shark taking a chunk out of his surfboard.

In response, all of the city’s northern beaches have been closed for at least 48 hours.

More than 30 SMART drumlines have been deployed along that stretch of coast, while shark listening stations and aerial surveillance have been ramped up.

Surf Life Saving NSW chief executive Steven Pearce said the “turbid and brackish” water created prime conditions for bull shark activity.

University of Sydney public policy expert Chris Pepin-Neff, who has written a book on shark policy, suggested Sydney Harbour’s 100-year-old sewage pipes were likely to blame.

The pipes overflow with just 20 millimetres of rain. More than 120 millimetres fell on Sunday.

“When the pipes overflow, when there’s more sewage, the bait fish eat the sewage and then the bull sharks come to eat the bait fish, and that draws them into the surface and into the shore,” Pepin-Neff said.

“Three shark bites in two days suggests to me there’s a unique environmental condition that is more than just the heavy rain.”

State officials are working to identify the species of shark involved in each incident.

-AAP