Emergency services winched two people from the surf at Wattamolla Beach, but one of them couldn't be revived, New South Wales Police said in a statement.
The pair had been fishing when swept off rocks on the outskirts of southern Sydney.
The second person, a 14-year-old boy, was in a stable condition in hospital.
It continued a run of fatalities over the holiday weekend, with the search ongoing for two other people still missing in the surf since Friday -- one off Sydney and one off Melbourne.
Surf Life Saving Australia (SLSA) issued a warning on Thursday that strong swells would create hazardous surf conditions along the coasts of New South Wales and Victoria.
Three people drowned on Friday in New South Wales, while a woman died and a man went missing after they were swept into the sea near Melbourne.
On Saturday, when swells were as high as 3.5 metres (11.5 feet) in some places, a fisherman was swept to his death off rocks in southern New South Wales.
SLSA had released data before the weekend showing 630 people had died at beaches without an active lifesaving service over the last 10 years.
Chief executive Adam Weir advised patrolled beaches should be prioritised for safety.
"We know that Aussies and visitors to our country like to go off the beaten track to enjoy camping, fishing and other coastal activities," Weir said in a statement.
"But these coastal locations can present dangers, some that you can see and some that you can't, which is why we have some simple advice: Stop, Look, Stay Alive."
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