Shark bites surfer in Australian state's fourth attack in 48 hours

Heavy rains stirring up murky waters have been blamed for the unusual spate of attacks in the state of New South Wales.

Surf Life Saving New South Wales said the turbid waters were ideal for bull sharks, urging people to stick to the pool until conditions cleared.

"We have such poor water quality that's conducive to bull shark activity," the water safety body told national broadcaster ABC.

"Just go to a local pool, because at this stage, we're advising that the beaches are unsafe."

A surfer on the state's Mid North Coast was lucky to escape with "minor" injuries after he was bitten by a shark while catching waves on Tuesday, Surf Life Saving New South Wales said.

It was the state's fourth recorded shark attack over the past 48 hours.

In one of those attacks, a shark bit a man's legs as he surfed at Manly, leaving him in a critical condition.

A few hours earlier, an 11-year-old boy escaped uninjured when a shark took a bite out his surfboard a little further north.

All of Sydney's northern beaches have been closed until further notice.

On Sunday afternoon, a 12-year-old boy was seriously injured as he swam at a beach on Sydney Harbour.

"It was a horrendous scene at the time when police attended. We believe it was something like a bull shark that attacked the lower limbs of that boy," said Superintendent Joseph McNulty, New South Wales marine area police commander.

"That boy is fighting for his life now," he told reporters on Monday.

Scientists suggested recent heavy rain had attracted bull sharks to coastal areas where rivers emptied into the sea.

"Sharks, especially bulls, are drawn to freshwater flushes to feed on fish and dead animals as they drift down from rivers," said Culum Brown from Macquarie University.

"Given the incredible rainfall we have had of late, the risk of encountering sharks is high. Stay out of the water till it clears."