Wollongong City Council lifeguards will extend their patrol hours on 12 popular beaches.

Temperatures are expected to rise significantly across much of the state, with the mercury expected to hit 38 degrees in Wollongong and Kiama and 40 degrees in Albion Park and Nowra.

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Flags will be up until 7pm at Stanwell Park, Coledale, Austinmer, Thirroul, Bulli, Woonona, Corrimal, Towradgi, North Wollongong, City and Port Kembla beaches, and until 6pm at Sandon Point.

Council's acting beach services operations manager Tim Jennett said beachgoers should hold off going for a swim until later in the afternoon.

"The weather forecast for Thursday is for some very unpleasant conditions and heat,'' he said.

"With heat expected to linger well into the late afternoon, we're going to keep our beaches open for an extra hour and allow people a longer opportunity to cool off.''

Mr Jennett said people should note which beaches had extended patrol hours, and to stick to patrolled beaches for an after-work dip.

"Please, if you're headed to the beach, swim between the red and yellow flags during patrol hours," he said.

"Our lifeguards are there to help keep you out of trouble and these flags mark the safest place on the beach for a swim.

"And, remember, if there are no flags up, then there is no swimming.''

Mr Jennett said lifeguards would also provide additional staffing support to Surf Life Saving Illawarra on Australia Day.

"If you're looking to head to the beach over the long weekend the advice remains the same,'' he said.

"Stick to swimming between the red and yellow flags and always follow the advice of the lifeguards or volunteer life savers.

"We're here to help keep you safe.''