NSW residents are being urged to drink plenty of water and swim safely as sweltering temperatures intensify.
The mercury is tipped to reach near or above 40C in many parts of the state on Wednesday, with temperatures in the north-west town of Bourke forecast to hit 45C.
Sydneysiders should experience a less extreme 38C, but temperatures in some western suburbs, such as Penrith and Liverpool, are expected to nudge over 40C.
A total fire ban has been declared for some areas, including the greater Sydney and greater Hunter regions.
"The best way to reduce the risk of heat-related illness is to drink plenty of water and keep your body as cool as possible, and avoid prolonged sun exposure by staying indoors in cool or air-conditioned facilities," the Bureau of Meteorology said in a statement.
Surf lifesavers are urging those who head to the beach to watch themselves and loved ones for signs of heat stress, such as dizziness, fatigue and headaches.
They also want swimmers to stay between the flags.
"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," a Surf Lifesaving NSW statement said.
While a wind change is expected to bring relief to those in southern and central coastal parts of NSW on Thursday, the heat is tipped to build again on Friday.