A "severe to extreme" heatwave will grip large parts of the country across the weekend, making conditions difficult for firefighters working to control catastrophic blazes in several states.

Key points: BOM forecasts indicate temperatures are likely to be more than 40 degrees Celsius in some areas during the heatwave

BOM forecasts indicate temperatures are likely to be more than 40 degrees Celsius in some areas during the heatwave The hot spell comes a week after Australia narrowly avoided breaking a new three-day heat record

The hot spell comes a week after Australia narrowly avoided breaking a new three-day heat record Firefighters are still working to contain life-threatening bushfires in New South Wales and South Australia

Bureau of Meteorology (BOM) maps show large parts of South Australia, Victoria and New South Wales are expected to experience temperatures in the high 30s to 40s over the weekend and into early next week.

Despite much of the country experiencing a brief reprieve over Christmas, the heat is currently moving through central-western Australia and is expected to reach the south-eastern parts of the country by Friday.

Victorian authorities are particularly concerned about three large bushfires burning in East Gippsland, which could affect communities from the Great Dividing Range to the coast.

It is feared the looming heatwave conditions will cause the fires to spread, and that dry lightning forecast from Friday to Monday could start more fires.

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The state's emergency management commissioner, Andrew Crisp, urged holidaymakers to reconsider any plans to visit the region's parks and forests, and those already there to have an evacuation plan ready.

BOM forecaster Richard Russell said the fire danger in Victoria would be very high across the entire state this weekend and was likely to be upgraded to severe on Monday when the heatwave peaked.

"On Monday, temperatures are expected to reach 40 degrees [Celsius] across the majority of the state, and 44C in the north-west including Mildura and Swan Hill," he said.

"We will see thunderstorm activity likely over parts of the state, especially in the north and east of the state.

"Very little rainfall is expected and unfortunately, yet again, we are facing that dry lightning risk and are looking at potential gusty winds."

In South Australia, the threat level from the devastating Cudlee Creek fire in the Adelaide Hills has been downgraded to an advice message — however, the Country Fire Service is warning residents to stay alert.

"Any localised winds, along with the forecast weather will increase the chances of fire outbreaks. Everyone needs to remain vigilant," the CFS said.

Flames and smoke from the Cudlee Creek fire in the Adelaide Hills. ( ABC News: Casey Briggs )

Regional areas are likely to be the hottest on Monday, with Oodnadatta in South Australia peaking at 47C, Swan Hill in Victoria expected to reach 43C, and Menindee in New South Wales forecast for 45C.

As for metropolitan areas, Parramatta in Western Sydney is expected to reach 39C on Monday and Tuesday; Melbourne is forecast to reach 41C on Monday; and the northern Adelaide suburb of Elizabeth could reach 41C on Friday, 40C on Saturday and 39C on Sunday and Monday.

Canberra is expected to reach 38C on Saturday and Sunday, before hitting 40C on Monday.

Tasmanians will not be spared from the heat — Hobart is predicted to hit 36C on Monday.

The forecast has prompted BOM NSW to warn people in affected areas to plan ahead and learn how to look after themselves and others in the heat.

The Australian Red Cross recommends people take cool showers, place a damp cloth on the back of the neck, stay indoors where possible, drink plenty of water, wear loose-fitting clothing and snack on cold foods such as fruit and salads.

Conditions easing — in some parts — before NYE

Current BOM forecasts show the heatwave beginning to ease in parts of South Australia, western Victoria and Tasmania by Sunday, with temperatures dropping into the mid-to-high 20s in most capital cities on New Year's Day.

However, eastern NSW and parts of northern WA will continue to face severe to extreme heatwave conditions up to Wednesday, January 1.

Dubbo is expected to be 42C on Tuesday and 40C on Wednesday, while further east, Muswellbrook is forecast to hit 44C on Tuesday before cooling slightly to 38C on Wednesday.

Sydney is likely to be milder, forecast for 33C on Tuesday and down to 26C on Wednesday.

In South Australia, where holidaymakers are expected to flock to the River Murray in the lead-up to New Year's Eve, police have launched a water safety operation.

"In particular we ask that people remember to wear their life jackets — it is important to put safety first," Superintendent James Blandford said in a statement.

The extreme heat could also increase the risk of falling tree branches, with several incidents already reported.

In one case earlier this week, a huge tree limb estimated to weigh several tonnes came down at Mitcham Reserve in suburban Adelaide.

A huge branch came down at the popular Mitcham Reserve in Adelaide. ( Twitter: @FrankPangallo )

Forecast comes during record-breaking December

The heatwave forecast comes just a week after Australia narrowly missed out on making it three in a row for its hottest-ever days, breaking temperature records in parts of Victoria, South Australia and the ACT.

Data from the BOM showed Tuesday, December 17 to be Australia's hottest day on record.

That day, the average maximum temperature across the country was 40.9C, breaking the mark of 40.3C set in January 2013.

Tradesmen in Victoria looked for ways to cool down due to recent record temperatures. ( ABC News: Billy Draper )

South Australia experienced four days of extreme heatwave conditions — the highest rating level — with catastrophic bushfires sparking on Friday.

In New South Wales, about 100 homes were lost in bushfires over the weekend, but cooler conditions since Monday have allowed NSW RFS to plan containment strategies.

The ongoing heat saw the national location record for December broken twice within the space of a few hours on Thursday — Eucla in Western Australia reached 49.8C before it was eclipsed by SA's Nullarbor weather station with 49.9C.

The old record had been set at Birdsville (49.5C) in 1972.

The records last week were not just broken, but surpassed by big margins.

Professor David Karoly, a leading climate scientist at the CSIRO, said the cause was clear.

"We're smashing the extremes by effectively a degree relative to the other cases because that's by how much the Australian temperatures have warmed," he said.

While visiting fire grounds in South Australia on Tuesday, Prime Minister Scott Morrison announced that volunteer firefighters who are also Federal Government employees will be given four weeks' paid leave in addition to existing entitlements to help contain the nation's bushfire catastrophe.