As a heatwave bears down on Australia's southernmost state, mainlanders who've been enduring extreme temperatures for the past few weeks may think the predictions don't sound too bad.

Key points: Tasmania's severe heatwave is due to culminate on Monday night with lightning which could start fires

Tasmania's severe heatwave is due to culminate on Monday night with lightning which could start fires The heatwave is being dragged from mainland Australia by an approaching cold front

The heatwave is being dragged from mainland Australia by an approaching cold front Severe heatwaves are less frequent in Tasmania and can put vulnerable people such as the elderly at risk

Some states have recorded prolonged stints in the mid-40s in recent weeks, and Australia experienced its hottest day on record earlier this month.

But when it comes to heat, it's all relative.

Over the next three days, the mercury in Tasmania will climb into the 30s and then soar towards 40 degrees Celsius or higher on Monday.

The heatwave is being dragged from mainland Australia by an approaching cold front.

The state's hot weather is expected to culminate on Monday night in a statewide lightning spectacle, a phenomenon now mostly feared for its propensity to spark bushfires.

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But as the Bureau of Meteorology's (BOM) Alex Melitsis explains, there isn't a particular temperature which has to be surpassed to declare a heatwave.

A heatwave is a period of three days or more when the maximum and minimum temperatures are unusually high for the location.

"It depends on the general climate of the place as well as the recent temperatures," Mr Melitsis said.

"We're more acclimatised to the lower temperatures so a hotter day here really has a big impact on us."

Temperatures that meet the criteria for a heatwave at the end of summer will also generally be hotter than the temperatures that meet the criteria for a heatwave at the beginning.

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The BOM categorises heatwaves three ways — low-intensity, severe and extreme.

Low-intensity heatwaves are the most common and people generally cope with them.

Severe heatwaves are less frequent and put more vulnerable people, such as the elderly, at risk.

The heatwave hitting Tasmania, which is also affecting the mainland, is in the severe and low-intensity range.

Extreme heatwaves are rare and put everybody at risk, even the healthy.

In the last 200 years, according to the BOM, severe and extreme heatwaves have cost more lives than any other natural hazard in Australia.

Tasmania's midlands, north hit harder

This weekend, the more intense heat will be felt in inland areas like Tasmania's Midlands, Upper Derwent Valley and Central Plateau, with a high fire danger and temperatures in the mid-30s expected.

"The Upper Derwent Valley and places like Campania and Richmond might see temperatures approaching 39. We may even see some areas hit 40 degrees," Mr Melitsis said.

While Hobart is headed for a sweltering 38C, Launceston residents could be in for more gruelling conditions.

The crowd at this year's Falls Festival in Marion Bay is set to endure top temperatures in the 40s. ( Facebook: Viridian Photos )

"Hobart is spared, I suppose, in that we're seeing some warm temperatures and then one massive spike," said Mr Melitsis.

"But places like Launceston will see temperatures into the 30s for three days, which is really quite remarkable."

Monday's lightning show is expected to be accompanied by some rainfall.

"However, any lightning over the really dry parts of the east and north is concerning," Mr Melitsis said.

There is a total fire ban in place for much of the state until 2:00am on Tuesday, after which cooler conditions are expected with average temperatures of around 23 degrees for January.