Tiffany Tarnbul, BBC correspondent in Sydney
Four shark attacks on humans have been recorded along the coast of the Australian state of New South Wales in less than 48 hours.
The latest case was recorded on January 20, when a 32-year-old man was injured after a shark bit his board on the Central North Coast, police said.
The man has since been released from the hospital.
Three more shark attacks have been reported in the past 48 hours.
All beaches in Sydney's north have been closed until further notice, police said.
The attacks followed days of heavy rain, which New South Wales Chief Inspector Joseph McNulty previously said may have created a "favorable environment for shark attacks."
Rain washes nutrients into the water, which can attract sharks closer to shore.
The attack on January 20th took place near the Point Plomer campsite, about 450 kilometers north of Sydney.
The surfer was “very lucky not to have sustained serious injuries,” said Steve Pearce, the organization’s chief executive officer.Surf Life Saving NSW".
"We strongly recommend that no one swims or surfs near river mouths because that is obviously an area where sharks congregate," Pierce said.
"If the water is dirty, I would think twice before getting in it," he added.
Another young surfer had similar luck at Dee Wye Beach in Sydney on January 19th, and was not seriously injured in the shark attack.
In contrast, just hours later, a shark attacked a 27-year-old in nearby Manly.
The injuries are such that his "life will be changed," officials said.
The day before, on January 18, a shark attacked and injured a 12-year-old boy at a popular beach in Sydney Harbour.
Watch the video: Man who survived a shark bite
Authorities believe bull sharks are to blame for the attacks.
Bull sharks, which live in both salt and freshwater, are “one of the few marine predators of this species that are potentially dangerous to humans,” officials say.
They are the third deadliest species of shark, according to the International Shark Attack File.
Last November, a woman was killed and a man was seriously injured after being attacked by a bull shark on a remote beach, also on the Mid-North coast of New South Wales.
Bull sharks are usually seen around Sydney in January and February - Australia's summer months - because "the water is at their preferred temperature," Dr Daryl McPhee, associate professor of environmental science at Bond University, tells the BBC.
Although Australia is a global hotspot for shark attacks, the chances of humans being attacked are still low.
Therefore, this series of shark attacks in New South Wales is considered a rare phenomenon.
"This series of shark attacks in such a short period of time is very unusual," McPhee said.
Police are advising people to avoid the water in New South Wales due to recent weather, which has reduced water quality and visibility.
"I would recommend that no swimming be done in the harbour or other river systems across New South Wales at this time," Superintendent Joseph McNulty told reporters.
Rain can concentrate fish that sharks prey on downstream in rivers and on beaches, McPhee said.
"Bull sharks do very well in murky waters after rain, when they are looking for food."
"Where there are fish, there will be bull sharks. Unfortunately, they are sometimes found on popular beaches," he added.
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