Queenslanders have flocked to shopping centres, air-conditioned clubs and swimming pools to escape the heat today.
The heatwave has hit but the weather bureau says temperatures have been erratic.
Forecaster Michelle Berry says Brisbane has not hit the predicted 39 degree peak today, but it has been the hottest December day in seven years.
"It got to 37 degrees in the city now and it's a bit all over the place at the moment, I don't think we can can it just yet," she said.
It was a different story on the Sunshine Coast where a longstanding record was broken.
"Tewantin has smashed its very, very old December record - it's had the hottest day since December 1901, 39.7 degrees, and the previously record was set in 1901 with 39.2," Ms Berry said.
The bureau says temperatures were eight to 10 degrees above the December average in the state's south-east today.
Tomorrow is predicted to be slightly cooler.
Power supplier Energex said power demand swelled as the heatwave took hold.
Energex spokesman Nathan Hatch said demand reached extreme levels before midday.
"We're very confident with the robustness of the energy network today," he said.
"It is going to get hotter and there is going to be a greater demand on the network but we're well and truly confident how the network is going to hold up."
Some schools banned outside play to keep students from overheating.
Hospital emergency departments were on stand-by for an influx of heat-related illnesses.
On the state's Sunshine Coast, hundreds of people headed to beaches to escape the heat.
The busiest spot on the coast appears to be Mooloolaba Beach where lifeguards said bigger-than-normal weekday crowds flocked to get some relief.
Lifeguard Nick Bolton said hundreds of people were there.
"It's the heat driving them into the water you could say," he said.
However, it was too hot for 86-year-old Jean Wilhelmsen who spent half-an-hour on Maroochydore Beach.
"Cold beer - one will do me," she said.
Crowds were up at shopping centres on the Sunshine Coast as people tried to stay cool.
Meanwhile, residents near the southern inland Queensland town of Miles are being told to finalise their bushfire plans as a large fire travels rapidly towards the town.
Queensland Fire and Rescue has issued a watch and act message for residents in Miles, Hookswood, Columbool and Burncluith.
Twelve fire crews and two water bombing aircraft are trying to contain the fast-moving blaze.
Further east near Toowoomba, a slow-moving fire at Ravensbourne has burnt through about 160 hectares of inaccessible land.
There is currently no threat to property.
Another fire in the D'Aguilar National Park has prompted firefighters to sound the town siren at Mount Nebo and Mount Glorious, but again there is no immediate threat to properties.
Backburning is underway near Mount Larcom to strengthen containment lines.
Residents in the area should keep their doors and windows closed.
Further north, fire crews have spent the afternoon battling a large grass fire near the Flinders Highway at Woodstock west of Townsville, however no properties are under threat.
© ABC 2012
12:42 EDT Yesterday, the majority of the Upper Western district in New South Wales experienced their first 40 degree day for spring this year.