Shark experts reveal 'disgusting' reason behind Sydney's three horror attacks in 24 hours

Surf cameras recorded the harrowing scene as a fellow surfer brought the wounded victim to shore, where beachgoers rushed to assist and paramedics administered CPR on the sand whilst a rescue helicopter circled above.

.Earlier on Monday, an 11-year-old boy escaped unharmed when a shark chomped on his surfboard at Dee Why, also on the northern beaches.

Families and surfers have now been told to keep away from the water for a number of reasons as Australia reaches the height of summer - but one may be more grisly than most.

Joel Nancarrow, who runs Hunter Shark Jaw Restoration, issued a stark plea for people to remain on dry land for the next week.

"The next seven days is the most dangerous time we will have seen in years, this isn't scare tactics, this is f****** real life danger," he said, adding: "This will not be the last one."

Higher temperatures in the water have created ideal circumstances for bull sharks, whilst murky conditions from recent storms are both arousing the predators' interest and limiting swimmers' ability to spot approaching danger.

Chris Pepin-Neff, an associate professor of public policy at the University of Sydney, pointed to a grim combination of heavy rainfall and untreated sewage flowing into harbour waters.

"After 20 millimetres of rain, the level of faecal matter and raw sewage that are in the harbour attracts bait fish and brings sharks toward the bait fish," he explained.

The professor called for authorities to put up public warnings advising against swimming for 72 hours following major storms, arguing the city bears responsibility for alerting residents to dangers posed by ageing sewerage infrastructure.

David Baxter, a shark expert with four decades of experience, said that bull shark activity typically increases alongside rising sea temperatures.

"The fact is that the conditions were just right for a species like a bull shark, we've got dirty water, a lot of the runoffs from the creeks and the drains, and that it stimulates the curiosity of sharks," he told the ABC.

He explained that sharks are essentially sensory organisms that taste and feel their environment through the water.

"And then they don't have hands, so they feel with their mouth," Mr Baxter said, noting that their razor-sharp teeth can inadvertently sever major arteries during such investigative bites.

Murky water, overcast skies, drizzle and baitfish activity all heighten the likelihood of dangerous encounters, he warned.

Former Prime Minister Tony Abbott has called on the NSW Government to reinstate commercial shark fishing in Sydney Harbour to solve the spate of attacks.

"There needs to be fewer sharks," he told news.com.au, later writing on social media: "We cannot put the protection of sharks ahead of the safety of people."