Adelaide smashed its January 26 record, with the mercury at 44.4C at 5pm - more than three degrees higher than the 41.1C set on the same date in 2006.
It was hotter still in the northern suburbs and even more so in the west and north of the state, with Ceduna recording an eye-watering 49.5C peak.
A cooler change is expected to bring a measure of temporary relief for coastal areas with milder conditions during the middle of the week, but fire danger and heat stress impacts remain extreme.
While no significant fires had broken out, strengthening northwesterly winds ahead of a change on Tuesday, combined with forecast extreme temperatures in the southeast, meant the danger was far from over.
Gusty winds would elevate the fire danger in both SA and Victoria, as well as the Central Ranges of NSW.
Adding to the complexity, thunderstorms were forecast for much of the region - but from high-based clouds, meaning any rain was expected to evaporate before hitting the ground.
It raises the risk of new fires from dry lightning.
"We're looking at temperatures that may challenge January or even annual records around the state tomorrow (Tuesday)," warned the Bureau of Meteorology's Jonathan Fischer.
The temperature is set to reach 45C in Melbourne on Tuesday, while the town of Renmark in SA's Murray Mallee is looking at a potential top of 48C.
Mildura is forecast to hit 49C, two full degrees above the previous all-time high of 47C.
"If you wanted to go back to when we last saw maximum temperatures of 44 degrees or above (in Melbourne), you'd have to go back to 2009 during Black Saturday," said the Bureau of Meteorology's Sarah Scully.
But she said the forecast maximums only told half the story.
"It's the overnight minimums that are contributing to this persistent heatwave event," she said.
SA State Emergency Service chief Robert Charlton said the overnight minimums increased the danger of heat stress.
"We talk about those that are vulnerable, but really we're starting to see everybody impacted by this," he said.
He warned the prolonged nature of the event, exacerbated by hot overnight temperatures, caused a build-up of fatigue and increased the risk of people making poor and possibly fatal decisions.
"We want everybody to be really aware that as we start to move into this and conditions start to ease, there's still cumulative effects that will impact people," he said.
Adelaide was expecting an overnight low of 31C, with temperatures remaining in the mid-30s for much of the night.
The scorching temperatures are being driven by northerly winds, funnelling blistering inland heat towards coastal areas.
SA Country Fire Service commander Ray Jackson implored people not to become complacent.
"Make good decisions, avoid risky behaviours, keep our communities and our firefighters safe," he said.
He said the state's 13,500 volunteer firefighters were ready to go at a moment's notice, with an large air tanker also on standby.
"Know your bushfire plan, be ready to action it and be prepared to leave early," he said.
34°C