Ambulance crews have responded to more than 400 heat-related incidents this week as the Adelaide heatwave extends into its fourth day.
The ambulance service says 259 people were taken to hospital as the city sweltered through four days with temperatures above 40C.
On Friday the mercury was forecast to hit 46C, which would be Adelaide's hottest December day.
A cool change was expected to sweep through the city late in the day bringing welcome relief and dropping the forecast maximum for Saturday to just 25C.
On Thursday, the west coast town of Nullarbor had SA's hottest ever December day with a top of 49.9C while Ceduna, on the Eyre Peninsula, recorded its hottest day in more than 80 years at 48.8C.
Adelaide soared to 45.38C also its hottest December day.
Other locations where December records were broken included Port Augusta on 48.5C, Wudinna on 48.4C, Roxby Downs on 47.3C and Minnipa on 47.2C.
The heat has forced the cancellation of events across the state, including Friday's Twilight Race at Morphettville and the Renmark Christmas Pageant.
Victoria heats up
Hanging out a load of washing in Victoria's scorching heat may seem ideal, but not to the state government who wants residents to turn off washing machines to prevent power outages.
As temperature in the state's capital set to hit 44C on Friday, Victorians are being urged to take as much pressure off the electricity grid as possible.
"The experts assure us that we have sufficient energy supply for today," Health Minister Jenny Mikakos told reporters.
"But it is sensible to not turn on your dishwasher and your washing machine and all your electrical appliances on days like today."
Melbourne's temperature will hover about 44C, while Swan Hill and Mildura may reach 47C - three degrees above the December record.
The sky has been blanketed in smoke from New South Wales, which is reducing visibility to about 1 km in some places, the weather bureau said.
The fire danger rating is 'very high' to 'extreme' across all regions with a total fire ban.
There won't be any reprieve from the heat until early evening before cool air at midnight.
"The cool change is still on track and is still later than anyone would want," Bureau of Meteorology senior meteorologist Keris Arndt said.
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"We will slowly cool down with the sun going down but we won't have any cool air until about midnight."
Nearly 500 firefighters are battling three uncontained fires in East Gippsland, the County Fire Authority says.
There are another 118 firefighters patrolling blazes at Somerton with a helicopter and dozer on hand, while more than 260 firefighters remain at the contained Tambo Crossing fire.
People are being urged to check on their neighbours, the elderly and expectant mothers in the extreme heat, and reminded not to leave children locked in cars on hot days.
"The temperature inside a car can double in just the space of a few minutes. If you leave a child or a pet in a car unattended, you will be putting their life at risk," Ms Mikakos said.
There have been 1491 call outs to people trapped in cars with the bulk for children under the age of 13, including babies, since November 30, Ambulance Victoria data shows.
Homeless people in the CBD will be given free pool passes and movie tickets to help them stay cool, the City of Melbourne states.
Public Transport Victoria said extreme heat timetables will be in place on multiple V/Line services and Melbourne Yarra Trams has advised services will be altered on Friday.
NSW braces for hot Saturday
NSW is expected to get some reprieve from the heat heading into Christmas - but first the state must endure another "dangerously hot" day across coastal areas.
The Bureau of Meteorology is warning that Saturday could be even hotter than Thursday, when two firefighters were killed and several others hospitalised during an exhausting day battling out-of-control bushfires.
BOM acting NSW manager Jane Golding said temperatures on Saturday would again rise into the mid-40s, with Penrith expecting a maximum of 47C and coastal areas hitting the mid-to-high 30s.
"That's dangerously hot weather," she told reporters in Sydney on Friday.
A gusty cool change is then expected on Saturday afternoon which will bring some relief for central and southern NSW but also create dangerous conditions on waterways and challenging fire conditions.
Smoke is also expected to build-up around coastal areas.
"Those southerlies when they come through (will) be close to gale force," Ms Golding said.
"The other thing with this system is it is bringing through some swell behind it, so dangerous conditions in the waters on the beach tomorrow as well."
Ms Golding said the good news was there was some respite on the way for NSW as the southerly change moved eastwards after reaching Sydney about 5pm.
That should bring cooler temperatures set to stick around for several days, with eastern NSW just needing to get through Saturday, Ms Golding said.
NSW Ambulance superintendent Brent Armitage said there had been an increase in heat-related call-outs on Thursday, with the organisation responding to a 95-year-old woman on the Central Coast and a 34-year-old man in western NSW with heat exhaustion.
Paramedics have also seen a 10 per cent increase in respiratory call-outs over the past week due to bushfire smoke.
"With a predicted heatwave and temperatures tomorrow, now is not the time to be complacent. The risk is very real," Mr Armitage said.
Surf Life Saving NSW chief executive Steve Pearce, meanwhile, said Saturday would be "one of the first major challenging days" for surf life saving this summer.
Mr Pearce urged swimmers seeking respite from the heat to head to patrolled beaches.