Bushfire warnings have been put in place for NSW, Victoria, Tasmania and South Australia as the Bureau of Meteorology warned of severe heatwave conditions this weekend.
Authorities have warned people not to leave children or pets in cars because they could die within minutes.
"People think they're going to be a minute and they're just going to rush in and grab something, and you're never a minute," Port Phillip Animal Hospital vet Dr Emma Prideaux told reporters on Saturday.
"(A pet's) temperature rises very quickly. They already run at a higher temperature than humans and we do see deaths."
Holiday-makers have been urged to stay on top of changing conditions in their area.
"Extreme weather, extreme heat is as dangerous, more dangerous than any other natural disaster," Agriculture Minister Jaala Pulford said.
NSW: Take care
NSW residents are being told to prepare for a scorcher of a weekend, with warnings to drink plenty of water and to take care in the surf, and prepare properties for the increased bushfire danger.
The Bureau of Meteorology has warned of severe heatwave conditions for much of the state across the weekend with the mercury expected to peak on Sunday.
Parts of NSW are tipped to reach 45C on Saturday including Menindee in the far west and Ivanhoe in the state's centre.
Most of the state will swelter through temperatures above 40C on Sunday, including Bulahdelah on the mid-north coast which is set to reach 45C.
Sydneysiders will avoid the worst of the heat with temperatures forecast to peak at 38C in the city on Sunday when Penrith could hit 45C.
"A NSW Health study published just last month found extreme heatwaves lead to a more than 10 per cent increase in both deaths and ambulance callouts," Dr Ben Scalley, Director of Environmental Health, said in a statement on Friday.
"They put a lot of strain on the body and can cause dehydration, heat exhaustion and heat stroke."
Surf Life Saving NSW has implored people heading to the beach to take care, given there have been 10 drownings since the beginning of December.
The RFS is warning residents of an elevated fire danger and wants people to prepare properties now.
NSW Health urged people to drink plenty of water but say they are prepared for a rise in deaths and emergencies during the heatwave.
NSW Police has reminded people it is an offence for children or pets to be left alone in a vehicle, and as the weather heats up it can be fatal.
A man stretches himself while taking a swim in the ocean pool at Bronte Beach on December 16, 2017 in Sydney, Australia. Source: Getty
"Despite the most recent warning in December, a dog died after being left in a car at Rose Bay, and in a separate incident a 32-year-old man was arrested and charged at a Guildford hotel after he allegedly left a six-year-old boy in a car," a police release on Saturday said.
Police say livestock and pet owners should ensure clean water is always available, shading is provided where possible and no animals are left in confined, unventilated areas.
Victoria: Stay inside
Victorians have been told to stay inside as parts of the state brace for the hottest day in nearly two years.
Melbourne and Geelong are forecast to reach 42C on Saturday, with winds of up to 40km/h.
Bendigo, Echuca and Avalon are set to climb to 43C, while Mildura and Warracknabeal could soar to 45C.
Extreme fire warnings are in place in the Mallee and Wimmera, with severe warnings in the southwest and centre of the state.
"This heat is a killer. It's going to be like a blast furnace tomorrow and you need to adjust what you do," Ambulance Victoria's state health commander Paul Holman told reporters on Friday.
"You need to take this seriously. You need to take care. So put off the sporting events, put off the outside events, stay inside."
Saturday will be Melbourne's hottest day since January 13, 2016.
A fire ban has been issued statewide, with extreme fire warnings in the Mallee and Wimmera, and severe warnings in southwest and central parts.
A cool change in the late afternoon to early evening could cause the temperature to drop 20 degrees within hours in Melbourne.
People in regional areas have been urged to have their fire plans ready, while travellers need stay on top of the conditions in their area.
A teenager cools off on a waterslide during a heat wave at Glenelg on January 13, 2014 in Adelaide, Australia. Source: AAP
Authorities are pleading with people to act responsibly throughout the day.
"A lot of people will be returning from holidays this weekend and we ask that they consider simple steps such as ensuring campfires are completely extinguished and cigarette butts are not thrown from car windows on the way home," Assistant Commissioner Debra Abbott said.
"Use of farm machinery and tools such as angle grinders and welders should also be avoided."
Anyone found guilty of recklessly or intentionally causing a bushfire faces a penalty of up to 15 years imprisonment.
Lighting a fire on a Total Fire Ban day can also attract a fine of around $37,000 or up to two years in jail.
Tasmania: Severe fire risk
Tasmanian authorities are bracing for severe bushfire conditions in parts of the state, with hot and windy weather expected.
Temperatures are tipped to reach the mid-30s in the Apple Isle's east and south on Saturday with wind gusts of up to 45km/h.
A severe fire risk has been forecast for the state's midlands, Upper Derwent Valley and south east.
More than 200 fire brigades are on stand-by across the state, along with specialist response teams and aircraft.
Tasmania Fire Service state fire controller Jeremy Smith said on Friday the combination of dry grass and forest fuels would make bushfires extremely difficult to contain.
A 24-hour total fire ban has been in place for King Island and northern and southern Tasmania since midnight on Friday.
Earlier this week, homes in George Town were threatened by a deliberately-lit bushfire.
Hobart is expected to reach 34C before a cool change arrives on Sunday.
Canberra: Summernats warning
With the mercury soaring this weekend, petrol-heads at Canberra's Summernats car festival have been urged to stay well hydrated.
An estimated 50,000 people are expected to go through the gates on Saturday to check out the burnout action and admire the street machines and hot rods.
The temperature is expected to reach 36 on Saturday and hit 38 on Sunday.
Summernats organiser Andy Lopez urged patrons to go easy on the alcohol in the heat.
"Take it easy, don't drink too much," he told reporters, adding there was free water on offer across Exhibition Park.
Saturday's program includes the national burnout masters eliminations as well as a fireworks spectacular and night concert.
Ocean swimmers make their way into the sea from the wall of the ocean pool at Bronte Beach on December 16, 2017 in Sydney, Australia. Source: Getty
South Australia: Recipe for fire
Parts of South Australia have been declared a catastrophic fire risk.
A catastrophic fire risk has been declared for SA's Lower Southeast, Upper Southeast, and extreme ratings have been issued for the Mid North and the York Peninsula.
Adelaide will have a top temperature of 41C while the mercury is forecast to reach the mid-40s in some regional and outback towns.
A strong cool change is also expected to sweep across SA during the day, reaching Adelaide mid-afternoon, with the changing conditions adding to the fire risk.
CFS chief officer Greg Nettleton said the combination of high temperatures, strong winds and low humidity was a "recipe for fire".
"High temperatures, low humidity, strong winds, fuel, it's all there," he said.
