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The Bureau of Meteorology has forecast daytime temperatures of up to 12degC above average and 10degC higher than usual at night from Monday to Friday.
An extreme heatwave is expected to sizzle across most of eastern New South Wales this week while the rest of the state will experience severe conditions.
In Sydney temperatures are expected to peak on Thursday and Friday, with much of the Sydney basin experiencing temperatures 10degC above average for this time of the year, a bureau spokeswoman told AAP.
Western Sydney suburbs will swelter with temperatures up to 43degC in Richmond and 45degC in Penrith on Friday.
Records have already started breaking with Borrona Downs in the state's northwest recording the highest minimum temperature ever in NSW with 34.6degC on Monday.
South Australia has had a scorching start to the week, with severe heatwave conditions across most of the state and the government declaring a "code red" to unlock extra funds to support homeless shelters.
Severe heatwave conditions are expected for most of SA between Tuesday and Thursday with parts of the pastoral district to hit extreme heatwave conditions.
The hot weather comes as the Tour Down Under cycling event gets ready to start in Adelaide on Tuesday and the Australian Open continues in Melbourne.
Tennis players will have cooler conditions than the cyclists but only just - Melbourne is forecast to experience temperatures in the low to mid-30s all week.
A widespread low intensity heatwave is also expected between Tuesday and Friday from central Western Australia to southern parts of the Northern Territory and southwestern Queensland, as well as parts of Tasmania and Victoria.