Millions of Australians face days of searing temperatures, nearing 50 degrees in some places, as a record-breaking heatwave covers much of the continent.

The heat will extend “from Kalgoorlie to Canberra” and intensify over Friday and into the weekend, rising to 18 degrees above average in some places.

It will also bring the first real fire fears of the season to a broad stretch of south-eastern Australia.

“Record heat is possible through inland parts of South Australia, north-western parts of Victoria and western parts of NSW, particularly on Saturday with numerous locations expecting to reach or exceed November temperature records,” weather bureau senior meteorologist Dean Narramore said on Thursday.

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The heat will peak in inland South Australia and western NSW on Saturday, with temperatures approaching the mid- to high 40s.

It will also creep into northern Victoria, where temperatures are forecast to top 40 degrees.

“As we move into the weekend, Sunday is looking like a really hot day across large parts of NSW, including the east coast – western suburbs of Sydney look to have back to back 40 degree days over the weekend,” Mr Narramore said.

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A southerly cool change is expected to bring some relief to coastal areas on Sunday. But north-east NSW and south-east Queensland can expect the hot days to run into the beginning of next week.

“We could see western parts of the suburbs in Brisbane see three or four days in a row at or above 40 degrees,” he said.

Adelaide is forecast to reach 40 degrees on Friday. But the real eye-watering temperatures will be felt away from coastal areas.

“Inland areas are looking like days of 45 to 48-degree heat, particularly once you get north of Port Augusta. That will be felt in Victoria later on Friday,” Mr Narramore said.

Melbourne will be spared the worst. It is forecast to reach the mid-30s on Friday before a cool change that night.

There will be little relief for northern Victoria, however. Murray River towns from Mildura in the west to Yarrawonga in the east are likely to hit the low 40s for many of the coming days.

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It will be even hotter in parts of western and central NSW – including Griffith, Hay, Narromine and Dubbo. They face days of temperatures in the mid to high-40s.

Some coastal areas will also reach the low 40s. Sydney is forecast to have a top of 39 degrees on Sunday.

The searing heatwave will also bring a high to severe risk of fires across much of NSW.

“This will increase the fire risk across much of state with Very High and Severe fire danger likely on the weekend. Plan and prepare now,” the NSW Rural Fire Service tweeted on Thursday.

The weather system is bringing hot winds from inland Australia and increasing the fire risk, particularly in the Northern Rivers region and northwest of the Great Dividing Range.

“It will also be quite windy so we are keeping a close eye on the fire danger which is associated with this warm, heatwave situation,” Ms Reid said.

“It is going to be quite a volatile situation.”