13/12/2016 7:00 AM AEDT | Updated 14/12/2016 6:31 AM AEDT

Heatwave Warning For South Eastern Australia As It Braces For A Scorcher

Wednesday night won't drop below 25C.

Summer has well and truly arrived. A heatwave has hit south-eastern Australia, with temperatures rising well into the 30s across three states.

According to the Bureau Of Meteorology, the mercury has reached 31.7C already in Sydney and is forecast to hit a sweltering 38C before the end of Tuesday and 37C on Wednesday.

Melbourne's temperature is set to rise to 34C on Tuesday, sitting around 32C at 2:45pm but Wednesday will provide some relief with an expected high of 20C.

Adelaide can also expect to hit 36C before dropping down to a much cooler 24C.

The BoM has issued a severe fire warning for Illawarra/Shoalhaven, Lower Central West Plains, Eastern Riverina, Southern Riverina and Northern Riverina in NSW. A total fire ban has been declared.

There's also a severe fire warning in place in Victoria's Mallee, Wimmera, Northern Country and North East.

In South Australia, the warning was issued for the North West Pastoral, West Coast, Eastern Eyre Peninsula, Lower Eyre Peninsula, Flinders, Mount Lofty Ranges, Riverland and Upper South East.

There is an extreme fire danger in the state's Mid North, Yorke Peninsula and Murraylands.

The Red Cross is warning that heatwaves can be dangerous, reminding people to keep an eye out for the vulnerable, The Herald Sun reports.

"Remember to check on your neighbours because older people, pregnant women, children, those with a disability and people taking medications are among those who are more at risk in a heatwave," the organisation said.

People are being advised to stay indoors and stay hydrated, as even the beach may not be enough relief from this absolute scorcher of a forecast.

NSW Health say the four key messages to keep ourselves and others healthy in the heat are to drinking plenty of water, keep cool, take of others and have a plan.

By the looks of these Instagram snaps, the sun hit parts of South Eastern Australia early on Tuesday with people preparing for a very, very warm and sunny day.

☀️ in DH today

A photo posted by @doverheights on

Well Good Morning Manly! 📷@lovemanly . . #hotelsteynemanly #manlybeach #manlylocal #northernbeaches #sydney

A photo posted by Hotel Steyne Manly (@hotelsteyne) on

It feels like summer!! I can almost smell the holidays 🤓

A photo posted by Missfire ✨ Australia (@missfirephotography) on