Sydney sweltered through scorching temperatures topping 40 Celsius (104 Fahrenheit) on Wednesday, with beaches packed, bushfire warnings issued and people urged to stay hydrated.

Some towns in the northwest of New South Wales state could reach a sizzling 47 degrees on Friday, the Bureau of Meteorology warned.

"We're gonna go, obviously Bondi Beach, have a dip in the ocean and cool off -- the best way you can do it," said Sydney resident Brenton Melville, 26, as he headed for the ocean.

Authorities issued a total fire ban for several areas across the state while Surf Life Saving NSW warned of an increased risk of dehydration as temperatures soared.

"Lifeguards and lifesavers have been extremely busy over the last few weeks and we are urging the public to do what they can to help lessen the load by taking some responsibility for their own safety," said Surf Life Saving NSW manager Andy Kent.

New South Wales has had a balmy summer, but it has been marred by tragedy with more than 20 drownings in backyard pools, waterways or the ocean since Christmas Day.