Sydneysiders are being urged to drink plenty of water and not leave children, the elderly or pets in cars with the mercury forecast to hit 36C - 14 degrees above the city's October average.
The Bureau of Meteorology NSW said 36C was also expected in Penrith in the city's west on Friday, 10 degrees above the October average.
Meanwhile, Moree in northern NSW will reach 34C, Bourke in the north-west has a maximum of 37C, while the Hunter region town of Cessnock has a maximum of 36C.
It will be the hottest day in Sydney since March 6, and BOM NSW forecaster Helen Kirkup said strong north-westerly winds would keep the cooling sea breeze away from the city and coastal areas until later on Friday.
"Those north-westerly winds will continue all day and that will keep the temperatures up right across Sydney," she told AAP.
"But Bondi has a top of 33 degrees, which means they should see a north-easterly later this afternoon."
Emergency services said people should keep cool and take care of those at risk of heat-related illnesses.
"Motorists are reminded it is never okay to leave children, the elderly, or pets unattended in a car - even with the air conditioning running. Not only is this dangerous, it can be deadly," NSW Police said in a statement.
Total fire bans are in place for the far south coast, Central Ranges, greater Hunter and in the state's north west.
NSW Royal Fire Service inspector Ben Shepherd said this fire season was particularly bad with more than 500,000 hectares burned throughout the state.
He said this was already double the 200,000 hectares burned throughout the last two fire seasons combined, with two lives lost and more than 100 homes destroyed.
"We need people to take note that this is no ordinary fire season," he told AAP.
"Without any significant rainfall on the forecast, there is the real potential for this to be very long, protracted and dangerous."
A maximum of 32C has been forecasted in Sydney on Saturday, but the mercury is expected to drop on Sunday with a maximum of 24C expected.
Australian Associated Press