Central and southern Queensland are in the grip of a heatwave, which is expected to peak on the weekend.
The Bureau of Meteorology (BOM) said temperatures were set to soar over the next four days.
Today the mercury hit 32.3 degrees Celsius degrees in Brisbane, 34.9C in Ipswich and 37C in Blackwater.
It saw Queensland Ambulance Service paramedics respond to 17 heat-related incidents across the state.
The heat will peak on Saturday, when Brisbane's temperature is forecast to rise to 36C, with predicted peaks of 39C in Ipswich and 40C at Biloela in central Queensland.
Senior forecaster Sam Campbell said the hot nights had already started.
"It only got down to around 24 in Brisbane city overnight [Tuesday]," he said.
"It was quite humid so you throw that in with relative humidity and it actually felt more like 27 degrees throughout most of the night."
A very high fire danger is forecast for the central highlands, coalfields, the Lockyer Valley, Maranoa and Darling Downs today and tomorrow.
The Rural Fire Service's regional manager Brian Smith asked central Queenslanders to avoid any activities that could start a fire, including the use of power tools and machinery.
"We also encourage everyone to limit the use of fire if possible," he said.
Heatwaves are defined as three or more days of high maximum and minimum temperatures that are unusual for the location.
A heatwaves ends when the mercury falls to more normal levels.
A warning has also been issued to large parts of NSW, where the temperature could hit 40C in Sydney today.
Watch for signs of dehydration, check on neighbours: QAS
Heat illness expert Dr Liz Hannah, from the Climate and Health Alliance, said international data showed that there were five times more extreme heat days than extreme cold days.
"Australia really has to get very sensible about understanding heat and protecting ourselves," she said.
"Probably changing things like expectations that people should continue working out in the heat and being more flexible.
"We don't really productivity to stop but we can't risk people's health."
She said people would cope better with the heat if they had good cardio-respiratory fitness, as blood vessels dilate when the body heats up and the heart has to work harder.
"Your heart has to push a lot harder and beat a lot faster to spread the blood around," she said.
"If the heart is weak, then you can go into heart failure.
"What we find is as it heats up, heat deaths and people who flock into emergency [departments], that really spikes up, definitely after a few days of hot weather."
QAS director of patient safety Tony Hucker said people will need to keep up their fluids, wear cool clothing, and keep out of the sun.
Signs of dehydration will include dark urine, headaches, abnormal behaviour, nausea and vomiting.
"Heat-related illnesses can sneak up on you when you really don't expect it," he said.
"If you've got elderly neighbours it's really nice to just go and knock on the door and make sure they're okay."ABC