Bust out that Hawaiian shirt and find yourself some fruity refreshments because southern Australia is set to get a taste of the tropics — but all is not calm in paradise.

Key points: High humidity has been predicted to combine with extreme heat over the weekend for south-east NSW and Tasmania

High humidity has been predicted to combine with extreme heat over the weekend for south-east NSW and Tasmania Severe thunderstorms with heavy rainfall has been predicted for parts of SA, Victoria and Tasmania

Severe thunderstorms with heavy rainfall has been predicted for parts of SA, Victoria and Tasmania Gusty winds have been predicted for north-east Victoria, south-east NSW and the ACT, creating challenging conditions for firefighters

The dew point — an indicator of how much moisture in the air — is about to soar thanks to a low pressure system dragging tropical moisture south.

It has been combining with high temperatures to bring dangerous heatwave conditions and sticky nights.

Philip Landvogt, senior meteorologist at the Bureau of Meteorology's national operations centre, explained: "Dew point is the indicator of the amount of moisture in the air," he said.

"So it's the temperature that the air needs to cool to in order to form cloud."

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Dew point, sometimes called dew point temperature, has been used to calculate what is called the relative humidity.

Relative humidity is a measure of the amount of moisture in the air as a percentage of the total amount of moisture that air can hold.

But given that hotter air can hold more moisture, relative humidity changes with the air temperature.

So in instances like this, when temperatures are very high, relative humidity can underrepresent how much moisture there is in the air, making dew point the better measure.

At the moment the dew point has been skyrocketing over southern Australia.

Dew point and humidity affect our body's ability to cool itself.

"If we sweat and the air is really dry that sweat evaporates off our skin and cools us down," Mr Landvogt said.

"But when there's more moisture in the air, that sweat does not evaporate as efficiently, so we feel hotter."

This moisture has been predicted to combine with extreme heat over the next few days, worsening the heatwave conditions.

Severe, extreme heatwave conditions

Adelaide only got down to a minimum of 29 degrees Celsius on Thursday night and the city's dew point was up to 21C by 10:00am on Friday.

"It's quite rare to have dew points over 20C this far south," Mr Landvogt said.

"It's been a very muggy morning [and] hard to sleep, I can imagine, in Adelaide last night."

Adelaide was forecast to reach 36C on Friday and remain sticky into Saturday, but by Sunday a cold front was expected to bring a south-westerly wind change.

This change will clear out the humid air and bring cooler, dryer conditions.

Melbourne was forecast to reach 43C on Friday with the dew point increasing as a cold front moves through, pushing the moisture into eastern Australia.

Canberra was forecast to reach 41C on Friday and Saturday, with the humidity increasing.

Temperatures in the low 40s are expected to be widespread for New South Wales, but sea breezes will keep things cooler along the coast.

Severe heatwave conditions have already been experienced across most of the south-east over the last few days and extreme heatwave conditions are forecast to persist for south-east NSW and surrounds.

Under such conditions even the healthy are at risk.

Categorising heatwaves Heatwave severity Low-intensity The level of heat expected is unusual, but injury to people is not generally expected unless inappropriate activities are conducted or sensible precautions are not undertaken Severe Vulnerable people are at risk of injury Extreme Likely to cause impact across multiple areas such as infrastructure, transport, energy, agriculture and both healthy and vulnerable people are at risk of injury Source: Bureau of Meteorology

Could we at least get some rain from all this?

Hundreds of millimetres of rain has already fallen over northern Australia over the last few days and that rain has started to move down over northern South Australia.

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"We expect rain to extend all the way from Oodnadatta, through Adelaide, Mount Gambier, across to Melbourne, and down into Tasmania as that system slowly moves through," Mr Landvogt said.

He said 20–80mm was possible through SA and maybe more with storms, which would be very welcome for the state, which missed out on last week's rain.

"It's going to be quite unstable. With that humid air mass you get quite a lot of instability and thunderstorms developing as well," Mr Landvogt said.

"There's a possibility of severe thunderstorms with heavy rainfall.

"Hail is also a possibility and damaging wind gusts."

It was only last week that locations up and down the east coast were hit by damaging hail, including Canberra, Melbourne, Sydney and the Gold Coast.

The increased winds and storms could again bring muddy rain, as experienced last week — keep an eye out for the warnings.

Fires continue to burn

A map published by the ACT ESA shows the potential spread of the Orroral Valley fire. ( Supplied )

Mr Landvogt said as the change moved through it would also bring gusty winds, although not quite as strong as last week.

With increased winds would come an increased fire danger.

"We've got severe fire dangers today right through western and central parts of Victoria and also the south-eastern parts of Tasmania," Mr Langvogt said.

"Tomorrow that will shift to north-eastern Victoria and south-eastern parts of NSW and also the ACT."

This has been a concern given there have still been many fires burning in these regions, including the large Orroral Valley fire on the fringes of Canberra — the ACT has been declared a state of emergency.

The moisture has been helping to keep the fire danger down in SA, with ratings expected to stay at and below 'very high' for the next few days.

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What about the tennis?

Conditions are expected to be oppressive at Melbourne Park until Sunday morning. ( AP: Andy Brownbill )

A possible late thunderstorm was predicted for Melbourne, so it is just as well Rod Laver Arena has a roof for tonight's men's semi-final in the Australian Open.

Weather conditions for the women's final on Saturday night are predicted to be muggy with a maximum of 33C and 10–40mm with storms on the forecast.

But things are expected to clear up in Melbourne on Sunday after an early shower or two, with a max of just 33C for the men's final.

The change has been predicted to move though Canberra late on Sunday.

Disappointingly, the rain has been forecast to deteriorate though the firegrounds of eastern Victoria and south-east NSW, but storms could return as the system has been predicted to move through northern NSW and south-east Queensland early next week.

While it may be tropical over the next few days, don't expect paradise.