As the first heatwave of the summer travels across the country, tomorrow is going to be blisteringly hot in most of southern Australia.

Temperatures in the high-30s and low-40s are set to hit large parts of Australia. 

Melbourne and Adelaide are set to swelter under 40 + degree heat today, as even Hobart’s mercury is forecast to hit 36 and Canberra is forecast to hit 34.

Victoria metro has cancelled services to some lines on Thursday and Friday in anticipation of the extreme heat.

Perth itself has found reprieve from the up to 40 degree heat during the Test match that saw Australia reclaim the Ashes, with a high of 28 today and 27 and storms forecast tomorrow.

Regional Australia has been hit hard with the Western Australian town of Esperance breaking the record for its hottest December day on record with a top of 45.3 degrees at 1.23pm.

The Bureau of Meteorology weather services assistant director Alasdair Hainsworth said to stay hydrated in the first heatwave of the summer.

“This is the first heatwave of the summer, so it’s important to stay hydrated and avoid prolonged exposure to heat,” he said.

“Fire dangers are expected to be Very High to Extreme across several states, and the public is urged to check these in their local area.”

Mr Hainsworth said the heat was caused by a large high pressure system in the Tasman Sea which was moving slowly eastwards, directing northerly-winds over much of Central Australia driving the heat in a south-easterly direction.

Luckily, in most cities a cool change is expected to hit in time for a weekend – in Melbourne dropping 18 degrees from 40 on Thursday to 22 and raining by Saturday.

“A trough of low pressure will approach southeastern Australia towards the end of the week, causing winds to increase ahead of the cool change forecast on Friday,” Mr Hainsworth said.

Most of South Australia is under extreme fire weather warnings today and a total fire ban, with a high of 43 expected.

More than 200 firefighters, assisted by water bombers, battled a scrub fire in the Adelaide Hills yesterday as temperatures hit 41 degrees in the city.

The extended hot weather has concerned emergency services which is warning people to take precautions against the heat.

SES Deputy Chief Officer Dermot Barry said people must be mindful hot weather can have a detrimental impact on their health and wellbeing.

“Identify family, friends and neighbours who are elderly or have special needs and may require additional care and attention during times of extreme heat.

“Make sure they have access to working air conditioners and fans and that their home is equipped to cope with hot weather.”

For those in the midst of festive season activities, SA Ambulance service operations manager Anthony Pryzibilla had a special warning.

“This time of year is also the festive season which can involve alcohol. Keep in mind that alcohol dehydrates the body so you should always ensure water consumption.”

Likewise in Tasmania, a total fire ban has been issued for southern part of the state as it braces for the first high fire danger day of the summer.

The heat is less extreme in the more tropical states. Sydney will continue its run of mild, warm weather with 28 forecast today, rising to 34 tomorrow.

However a heatwave is set to hit parts of New South Wales in the lead up to Christmas.

Likewise Brisbane is expected to have cloudy weather in the high 20s and low 30s until next Tuesday, where the weather is forecast to spike to 35 degrees.

In the north Darwin has seasonably hot and sticky weather in the mid-30s with afternoon storms.