Parts of Sydney have broken the 40-degree barrier and swathes of western NSW, South Australia and northern Victoria are baking through even higher temperatures.
In the first heatwave since the catastrophic 2019-20 bushfire season, parts of inland NSW are set this weekend to hit 45C.
Most of Sydney - including the CBD - recorded temperatures of 40C on Saturday as strong northwesterly winds held back the sea breeze.
The highest-recorded temperature was at the airport, at 41.7C.
Bureau of Meteorology manager Jade Golding said November records for warmest overnight minimums in some parts of NSW likely tumbled on Friday night, while daytime records could also fall this weekend.
NSW and Sydney will swelter until at least late on Sunday afternoon, when a gusty southerly comes through and cools temperatures for Monday.
"It'll be a really long, uncomfortable night (on Saturday) and then a really long hot day and then a really windy southerly change," Ms Golding said.
"The body doesn't really get much respite, it's quite hard."
Temperatures across NSW will then spike again on Tuesday as heatwave conditions envelope inland areas. Southeast Queensland will also likely experience elevated temperatures from Monday.
Relief from the heat may not arrive again until Thursday.
Ms Golding said the weekend swelter would likely spur bushfire concerns, with the Rural Fire Service forecasting severe danger across southern NSW regions on Saturday including the Riverina.
She said fires would be fuelled by strong grass growth over a rainy winter.
The RFS has also issued a total fire ban for most of eastern and northeastern NSW for Sunday, including Greater Sydney, Illawarra, the Hunter and north coast. No fires will permitted out in the open.
One 416-hectare blaze at Myrtle Park, east of border town Deniliquin, has been downgraded to "advice" level after being brought under control on Saturday afternoon. The RFS' large air tanker was on scene to assist firefighters.
RFS Deputy Commissioner Peter McKechnie on Friday urged people to have fire plans ready and prepare properties.
"This is the first time since the devastating season last year we've seen widespread elevated fire danger," he said.
Last summer's bushfires destroyed 2476 homes and claimed 26 lives.
NSW Ambulance Inspector Kay Armstrong told reporters on Saturday paramedics were dealing with increased call-outs and urged people to avoid the worst of the heat.
South Australia followed up a scorching Friday with a similarly warm Saturday, with temperatures hitting 46C in Port Augusta and Coober Pedy.
Northwest Victoria is also feeling the heat, with temperatures in Mildura on Saturday reaching 45C.
Australian Associated Press