AS temperatures soar across Australia, Canadian YouTuber Frankie MacDonald and the internet have the hot tips to keep you cool

A COOL change has arrived across southeastern Australia after scorching temperatures on Tuesday sent people flocking to beaches and air-conditioning, as elsewhere bushfires scorched vast tracts of land.

After sweltering through 40-plus temperatures and bushfires on Tuesday, much of southeast Australia has woken to balmy temperatures in the low 20s, and they are not expected to rise significantly from there.

Residents in Victoria's Mount Hotham were stunned when snowflakes began falling early this morning as the cool change hit.

With a wind chill factor of -3C, the flakes fell but it wasn't cold enough for them to settle on the ground.

Meanwhile, Sydney is today forecast to be 25 after hitting 41 on Tuesday.

The hottest place was Oodnadatta in South Australia at 48.2 degrees, but temperatures well into the 40s were also recorded in South Australia, Western Australia, NSW and Queensland.

It was so hot in Oodnadatta, the service station in the outback town stopped serving petrol.

And in Sydney, it was so hot overnight hundreds flocked to Bondi Beach for a midnight swim to escape the heat.

Weather forecasters believe Tuesday may have been Australia's hottest day on record but they won't know until Wednesday morning then they calculate the average.

So far, Monday remains the hottest day since records began with an average maximum temperature across Australia of 40.33 degrees, beating the previous record of 40.17 degrees set in 1972, the Bureau of Meteorology's David Jones told AAP.

Average maximum temperatures have risen above 40 degrees only three times in recorded history.

"We had the hottest day on record for Australia (on Monday) and today it looks like we may well go better again," Dr Jones said.

Not everyone could evade the oppressive heat, with NSW paramedics responding to a spate of heat-related casualties as Sydney recorded a peak temperature of 42.5 degrees.

Much of NSW also broke the 40 degree barrier, with Bega on the south coast the first town to hit the mark at 11.30am (AEDT).

Michael Clunie, who works for Forests NSW, was ordered out of the Bomballa State Forrest in the morning when the temperature topped 41 Celsius in Bega.

''It's a relief,'' he said of the southerly.

''People were worried about the whole area.''

Jo, a local business owner in the town's main strip, said takings were down by 75 per cent when she went to the bank after closing shop.

''The people just didn't come out or were evacuating and a lot of people who work in offices in Bega left because they were worried about their homes,'' she said.

Boss: 'It's too hot, go home!'

Heat related call-outs rose across the state, with 17 patients treated for heat exposure in Sydney alone, eight of whom were aged 70 years and over.

Many took refuge from the NSW heatwave by staying indoors, while the more adventurous headed to the beach.

Icy treats for hot zoo animals

While workers luxuriated in their air-conditioned offices, holidaymakers found other ways to cool down.

Extra surf lifesavers volunteered today at beaches that are not normally patrolled on weekdays.

In Melbourne children were being rescued after being locked in cars with the windows closed, as the temperature inside exceeded 50 degrees.

Ambulance Victoria operations manager Paul Holman said since last Friday, when the temperature exceeded 40 degrees in Melbourne, between seven and 12 children have been found locked in cars each day, with passersby alerting authorities.

Some of the children have been hospitalised.

"I am a little bit astounded that we're still dealing with this and particularly that we are dealing with it on a day of, you know, 40-degree heat," Mr Holman told reporters today.

"It's like putting a gun to your child's head."

he ACT Ambulance Service treated 19 people for heat related illnesses while in Adelaide, the Zoo was taking special steps to keep the animals cool including ice blocks filled with meat, fish or fruit as well as providing water misters and wading pools.

In Queensland, Bureau of Meteorology senior forecaster Michael Knepp said a trough behind NSW's hot temperatures is making its way to southern Queensland, which will trigger a rise in temperature across the state next week.

Brisbane can expect a maximum of 36 degrees tomorrow, Alice Springs will have a scorching 43 degrees, Darwin is heading for 32 and Perth 30.

But there will be a cooler respite in other capitals.

Sydney should have 25 degrees, Melbourne 20, Adelaide 25, Canberra 28 and Hobart a cool 16.