The Bureau of Meteorology is forecasting Sydney's city centre to reach 32 degrees Celsius on Sunday, while Penrith in the west is tipped to hit 40C and Richmond and Blacktown should reach 39C.
Temperatures in parts of the state's west are predicted to top 40C, including in Forbes, Griffith, Ivanhoe and Cobar.
It comes after NSW sweltered through a hot Saturday, leaving more than 30 bush and grass fires burning across the state on Sunday morning.
A fire at Craigs Road in Curraweela, in the southern tablelands, was being controlled but had burned through 3689 hectares, destroying property and livestock.
Another large fire was burning out of control at Yarra Station near Mount Hope in the state's west.
The temperature in Canberra is predicted to hit 36C on Sunday, while Perth is expected to reach 33C and Brisbane 32C.
Meanwhile, Victoria is in for a cooler day after temperatures reached the high 30s and low 40s on Saturday.
Residents along the Great Ocean Road in the state's southwest were on high alert for much of the day, with a watch-and-act message in place for a blaze near Kennett River although the warning was downgraded in the afternoon.
Several other fires reached advice level before a cool change passed through the state on Saturday evening.
Victorian temperatures should hover about the low to mid-20s for the rest of the week.
20°C