Heavy rain and high surf 
By Alasdair Pal
Emergency services were called to a beach in Manly on Monday evening following reports a surfer had been bitten by a shark, New South Wales police said in a statement.
Max White, an eyewitness, said another surfer had kept the man alive using his surfboard'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, as well as all the other people around us. Everyone was involved," he told state broadcaster ABC.
Paramedics treated the man for serious leg injuries before taking him to hospital in critical condition.
All beaches in the Northern Beaches, a council area straddling the city's northern coastline, will remain closed until further notice, police said.
The attacks follow days of heavy rain that ran off into the harbour and beaches around the city, creating ideal conditions for the bull sharks suspected to be behind some of the attacks. The species thrives in brackish water.
"If anyone's thinking of heading into the surf this morning anywhere along the Northern Beaches, think again," Steven Pearce, the chief executive of Surf Life Saving NSW told reporters on Tuesday.
"We have such poor water quality that's really conducive to some bull shark activity."
On Monday, a 10-year-old boy escaped unharmed after a shark knocked him off his surfboard and bit a chunk out of it. A day earlier, another boy was left in critical condition after being bitten at a city beach.
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.
(Reporting by Alasdair Pal in Sydney; Editing by Himani Sarkar)