Severe heatwave forecast for NSW, with Sydney's west set to reach temperatures in the low 40s

The first heatwave of the summer will reach its peak across New South Wales today, with temperatures above 40 degrees Celsius likely to span from Western Australia's north-west all the way to Parramatta.

The heatwave has also sparked an extreme fire danger warning for large parts of NSW.

But reprieve is around the corner — at least in NSW — with a cold front set to bring a cooler change by the end of the weekend.

It's the first taste of what's likely to be a hot summer for most of the country, according to the Bureau of Meteorology's long-range outlook.

Heatwave moves across the country

Severe heatwave warnings are in place for large areas of the country, including parts of the NSW coast from east of Cooma to Newcastle on Saturday, as well as Sydney and Wollongong.

Bureau of Meteorology senior forecaster Jonathan How said parts of the NSW coast could see temperatures up to 15C above average.

"Saturday will be the peak across Sydney and eastern New South Wales," he said.

"Sydney is getting to 37C in the city, the low 40s in the west.

"So it's a very hot day and there are very hot conditions all the way up the coast."

Meanwhile, the highest temperatures are expected in WA's Kimberley region, where extreme heatwave conditions are set to linger into next week.

Fitzroy Crossing is forecast to reach 46C on Sunday and Monday, according to the BOM.

"We do see some storm activity picking up in the middle of next week," Mr How said.

"That's helping to dissipate the heat, but no sort of big monsoon burst is coming in."

Extreme fire danger for Sydney, Hunter, Illawarra

There are warnings of extreme fire danger for large parts of New South Wales, with the BOM also warning of the risk of dry lightning across parts of the state on Saturday.

Total fire bans are in place in nine areas, including in Greater Sydney, Greater Hunter and the Illawarra/Shoalhaven, across the weekend.

Rural Fire Service spokesman Ben Shepherd said a bushfire near Bulahdelah on the Mid North Coast was expected to present a challenge, with winds expected to shift back to the north-west.

On Friday the blaze was burning at an emergency warning level and closed the M1 to traffic.

There were also concerns that the forecast conditions could fuel a blaze at Milsons Gully, near Sandy Hollow in the Upper Hunter.

Bursts of short but sharp heat have been spreading across the entire country all week, bringing Perth's hottest start to summer on record with a Monday high of 39C.

By Thursday, a north-westerly airstream had carried the wave of heat to south-east states — Adelaide and Melbourne both reached 35C, their warmest days since March.

Cool change and storm risk

Saturday will be the last of the hot weather for southern Australia for now.

Milder conditions have already arrived for WA and Victoria, with a cold front forecast to bring showers to large parts of Victoria and Tasmania.

That includes the chance of snow down to 700m in Tasmania during Saturday evening.

Severe thunderstorms are also possible on Saturday for eastern parts of Victoria, and parts of the south-coast New South Wales up to the Illawarra region, according to Mr How.

By Sunday NSW will be in for a cooler change.

"We do see a cool change push across Sydney Sunday morning that will bring cool temperatures and relief to the Central Coast … up into … northern inland parts of New South Wales," Mr How said.

"But southern Queensland — it looks like the cold front seems to sort of stop there."

Mr How said severe storms were possible from Saturday afternoon and evening for the northern parts of the state, with the risk of wind and pockets of heavy rainfall from "Newcastle to about the Tablelands".

The peak risk is on Sunday.

"Sunday looks to be sort of a day of more widespread severe thunderstorm activity," he said.

"So we anywhere from about the Hunter, up towards Lismore and inland towards Downs, excluding Tweed Heads and Brisbane, could see damaging winds and large hail."

Hot summer fuels bushfire risk

This week's heat is likely to mark the start of a hot summer ahead for Australia.

The BOM's long-range outlook for December to February is favouring warmer-than-normal temperatures across almost the entire country, during the day and the night.

Before: The chance of above average maximum temperatures for the summer months across the country.. . After: The chance of above average minimum temperatures for the summer months across the country.. .

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It includes a double to triple the normal chance of "unusually" warm summertime maximums across Victoria, Tasmania, eastern Queensland and northern WA and the Northern Territory.

Large parts of northern Australia are also very likely to see unusually warm nights during the season.

BOM senior climatologist Simon Grainger said this was likely to increase the risk of heatwaves this summer.

"Heatwaves can be often very warm at night. So it's all areas where there's increased chance of daytime and night-time temperatures," he told the ABC last week.

The forecast, combined with long-term dryness across some parts of the landscape, has prompted fire and emergency services to warn some parts of the country could transition into a "highly flammable" state in the season ahead.

In its seasonal bushfire outlook, the National Council for Fire and Emergency Service (AFAC) identified increased risk of fires across parts of southern WA and NSW.

Large parts of Victoria are also highlighted as having an increased fire risk later in the season, following long-term rainfall deficiencies in the region.