SYDNEY — Dozens of beaches along Australia's east coast, including in Sydney, closed on Tuesday after four shark attacks in two days, as heavy rains left waters murky and more likely to attract the animals.

Beaches around Port Macquarie, around 400 north of Sydney, were shut after a man was bitten while surfing earlier in the day. He remains in hospital in a stable condition, health authorities said.

The 39-year-old man "sustained a wound to his chest" after a shark bit his board on the Mid North Coast on Tuesday morning, police said.

The attack took place near the Point Plomer campground, about 450km (279 miles) north of Sydney.

It follows three other attacks in Sydney over the past two days. All beaches in Sydney's northern area will remain closed until further notice, said police.

The attacks follow days of heavy rains, which NSW Superintendent Joseph McNulty had earlier said may have created a "perform storm environment" for shark attacks. Rain flushes nutrients into the water, which can draw sharks closer to shore.

"If you're thinking about going for a swim, think of going to a local pool because at this stage, we're advising that beaches are unsafe," Steven Pearce, the chief executive of Surf Life Saving New South Wales (NSW) told reporters on Tuesday.

"We have such poor water quality that's really conducive to some bull shark activity."

The closures come in the middle of the Southern Hemisphere summer, when beaches across Australia are normally packed with locals and tourists.

On Monday evening, emergency services were called to a beach in Sydney's Manly after reports a surfer in his 20s had been bitten by a shark. Eyewitness Max White said another surfer had kept the man alive using his board's leg rope as a makeshift tourniquet to stem the bleeding.

"He was breathing, but he was unconscious, and we just ... tried to keep him awake," he told state broadcaster ABC.

Paramedics treated the man for serious leg injuries before taking him to hospital in critical condition.

Also on Monday, a 10-year-old boy escaped unharmed after a shark knocked him off his surfboard and bit a chunk out of it, while a day earlier, another boy was left in critical condition after being bitten at a city beach.

Sharks do not normally attack humans, but the turbid water reduces their visibility and raises the risk of them bumping into something, at which point "they defensively or curiously bite and then bite again", Chris Pepin-Neff, an academic and expert on shark behaviour, wrote in a column in the Sydney Morning Herald newspaper.

Heavy rain also increases sewage runoff, drawing in bait fish that sharks feed on, he added.

Australia sees around 20 shark attacks per year with fewer than three of those being fatalities, according to data from conservation groups. Those numbers are dwarfed by drownings on the country's beaches.

Authorities believe bull sharks were involved in most of the recent attacks.

Bull sharks, which can be found in both fresh water and salt water, are "one of the few sharks that are potentially dangerous to people", the Australian Museum says. They are the third deadliest shark species, according to the International Shark Attack File.— Agencies