Temperatures are expected to reach the mid-40C range in some areas.
Sydney’s east is expected to reach a top range of between 30-33C during the long weekend, but temperatures will near 40C in the west, and Penrith could crack 41C.
Temperatures of 40C or higher are also expected in the west of the state and on the south coast.
Melbourne will also dodge the 40C limit, with temperatures predicted to peak at 38C on Sunday.
However, further north, more intense and more extended heat is expected, with Mildura in the north-west forecast to reach 45C on Sunday, along with consecutive days of 40C-plus weather.
Adelaide is set for its first 40C day of 2021, with a forecast of 41C on Sunday.
Regions in the north of the state are headed for at least five consecutive days of temperatures topping out in the low- to mid-40C range, Weatherzone reported.
Queensland, Tasmania, Western Australia and the Northern Territory are unlikely to be affected by the heatwave.
Brisbane is forecast to reach 30C on Saturday and Sunday, while Darwin will top out at 33C.
In Perth temperatures will be higher with a maximum of 36C, but Eucla in the state’s south-east will suffer a heatwave, peaking at 42C on Saturday.
Temperatures in Tasmania’s north are expected to hit the mid-30C range.