Temperatures pass 40C as heatwave continues across four states

Temperatures have passed 40 degrees in parts of Victoria as people living in the country's south sweat through a summer heatwave.

Melbourne reached 37.2 degrees at 4pm making it the hottest day of the year, meanwhile Adelaide reached a top of 39.6 degrees.

Heatwave warnings have been issued across South Australia, Victoria, Tasmania and New South Wales.

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However, the hot weather won't last much longer, with the Bureau of Meteorology advising a cool change will sweep through mid-week.

In parts of Adelaide, temperatures have also soared above 40 degrees today, sparking a total fire ban across the regions, including Lower Eyre and Yorke Peninsulas.

The ban will be in place until midnight tomorrow when temperatures are expected to drop back into the mid 20s.

Heat and thunderstorms expected for Victoria

Meanwhile, Victoria has experienced days of scorching temperatures driving thousands of people to the state's waterways and beaches.

Today was no different, with towns in the state's north west to exceed 40 degrees in the hottest day of the year.

Suburbs in Mallee in the north-west near the NSW-Victoria border, Wimmera, Northern Country and North Central all saw blistering temperatures today.

There is also a severe thunderstorm warning for heavy rainfall for parts of East Gippsland, West and South Gippsland and North East Forecast Districts.

The warm weather has sparked a warning from Ambulance Victoria to not leave your children or pets in cars on warm days.

"Heat can cause illnesses such as heat cramps and heat exhaustion or lead to heatstroke which is fatal in up to 80 per cent of cases – however heatstroke is entirely preventative," an Ambulance Victoria spokesperson said.

"Those most at risk are older people, young children and people with a medical condition. A child's body temperature rises three to five times faster than an adult."

In November, paramedics were called out to 113 cases of locked cars.

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The heatwave sweeping across Victoria's north-west will also drove up temperatures in New South Wales' south.

"Maximum temperatures in the mid- to high-thirties are expected over southern inland parts, with minimum temperatures in the low to mid twenties," the bureau said earlier.

"The heat will peak over most parts on Tuesday as maximum temperatures reach forty over southern inland parts."

Sydney also reached a top of 28 degrees today.

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Tasmania isn't exempt from the sweltering conditions with Hobart hitting a high of 30 degrees today.

The bureau said coastal areas will peak at the low to mid-thirties.

The Central North, Central Plateau, western suburbs, Midlands, Upper Derwent Valley and some of the south east will all reach above 30 degrees.

There is also a severe weather warning in place for much of the state.

The bureau has advised that strong north to northwesterly winds from a cold front might have the potential to be damaging for parts of Tasmania tomorrow.

Heatwaves can be particularly dangerous for older people, babies, children, pregnant and breastfeeding women, people with medical conditions and those who are unwell.

"Seek a place to keep cool, such as your home, a library, community centre or shopping centre," the bureau advised.

"Close your windows and draw blinds, curtains or awnings early in the day to keep the heat out of your home.

"If available, use fans or air-conditioners to keep cool."