Twenty Sydney beaches have been closed indefinitely, following a third shark attack within 24 hours. 

A surfer is in a critical condition with serious leg injuries, after being bitten at North Steyne Beach at Manly in Sydney's north last night.

It happened a matter of hours after an 11-year-old surfer had his board bitten by a shark a few kilometres away at Dee Why - and a 13-year-old boy was attacked while swimming on Sunday.

Australian correspondent Murray Olds says experts have raised concerns about adverse weather conditions luring in the sharks, putting people at risk.

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