A surfer at a popular Manly beach avoided entering the water minutes before a serious shark attack after being warned of the danger by his mates.

Max White surfs at North Steyne Beach on Sydney’s Northern Beaches almost every day, and he had planned to do the same yesterday afternoon.

However, when he told his mates, they urged him not to go in the water.

Max White only avoided entering the water after a warning from his mates. (Nine)

“I walked over and said to them, ‘I’ll go for a surf’. And they looked at me and started shaking their head,” he told 2GB’s Ben Fordham.

“They go, ‘no, don’t go out, you’re going to get done by a shark’.”

White said just 20 minutes later, a surfer was mauled by what is believed to be a bull shark, suffering serious leg injuries. He is now fighting for life in hospital.

It was the third shark attack in Sydney in just 26 hours, with a 12-year-old boy also fighting for life after being bitten in Vaucluse.

Another boy had a chunk taken out of his surfboard in Dee Why.

Wet weather in Sydney has created optimal conditions for shark attacks, with waters murkier due to the recent downpour.

More than a dozen beaches across the city have now been closed as a result of the attacks.

White said he recalled seeing blood in the water immediately after the attack. (Courtney Robinson)

White said he was still on the beach when the attack happened, saying two French backpackers raised the call for help.

“[They] ran up to us and started screaming, ‘in the water, there’s a guy, he’s bleeding out,'” he said.

“We looked out, and as he went over a wave, you could just see the pool of blood in the water.

“No matter how murky the water was, the wave was red.”

Third shark attack in two days prompts beach closures

The Manly attack is the third shark incident in Sydney waters in just two days.

On Monday, a suspected bull shark bit an 11-year-old boy’s surfboard at Dee Why Beach.

Meanwhile, 12-year-old Nico Antic remains in a critical condition in hospital after a shark attack in Sydney Harbour on Sunday.

Authorities have issued urgent warnings for swimmers and surfers to stay out of the water, with every beach across Sydney’s northern coastline now closed.

Surf Life Saving NSW CEO Steven Pearce told Sunrise the response was unprecedented.

“The beaches are closed for your safety. The water is unsafe at the moment because of the turbidity and the murkiness of the water,” Pearce said.

“Do not enter the water for swimming or surfing for the next 48 hours. This is unprecedented to have this amount of shark activity incidents in the 24 hours.”

Despite the warnings, some surfers were spotted in the water on Tuesday morning, with jet skis patrolling and drones monitoring conditions across 10 beaches.

The Westpac Lifesaver Rescue Helicopter has also been deployed, while the Department of Primary Industries and Regional Development has installed 34 drumlines along the coast.

Likely reason for spike in shark attacks

Surf Life Saving NSW CEO 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 sewerage pipes were likely to blame.

The pipes overflow with just 20mm of rain, far less than the 127mm Sydney received 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,” Associate Professor Pepin-Neff told AAP.

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

Expert warns conditions remain dangerous

Shark expert Professor Robert Harcourt told Sunrise on Tuesday the cluster of attacks as “a bit of a shocker”.

“We have had clusters of shark attacks like we did up the north coast a few years ago and then in West Australia a few years before that,” he said.

“But yeah, three in a row, for those of us who surf, it’s definitely a bit of a shock.”

He believes bull sharks are responsible, drawn closer to shore by warmer waters and recent heavy rainfall.

“They feed where there’s been influxes of heavy rain because that moves their food around and they follow the food,” he said.

With more rain forecast, Professor Harcourt urged swimmers to remain out of the water.

“Until the water clears up, it’s still turbid and full of sort of a mixture of silt and fresh water and salt water, keeping out of the water until it’s clear and you can see what’s going on, is probably a really wise move,” he said.

Authorities say beaches will be reassessed in 48 hours, with reopening dependent on water clarity and safety conditions.

– With AAP