Australia swelters through heatwave

By AAP Newswire

Parts of Australia are sweltering through the start of a week-long heatwave as temperatures rise into the 40s in some regions.

The Bureau of Meteorology has forecast daytime temperatures of up to 12C above average and 10C higher than usual at night from Monday to Friday.

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.

"Agencies across Adelaide will extend their daytime operating hours over the next few days to ensure people sleeping rough are keeping cool and well hydrated during the extreme heat," Human Services Minister Michelle Lensink said.

Further north, Marree soared above 45C on Monday with temperatures in the high-30s to the mid-40s expected until at least Thursday.

"Some respite from the heat will come as early as Friday when a vigorous cold front moves through the state," senior meteorologist Dean Fgarboffa told AAP.

South Australian Premier Steven Marshall has backed calls from health and safety experts for people to take care by staying out of the heat as much as possible.

The hot weather comes as the Tour Down Under cycling event gets ready to kick off in Adelaide on Tuesday and the Australian Open fires up in Melbourne.

Tennis players will have cooler conditions than the cyclists but only just - Melbourne is forecast to experience temperatures in the low to medium 30s all week.

An extreme heatwave is expected to sizzle across most of eastern NSW this week while the rest of the state will experience severe conditions.

Sydney temperatures are expected to be above 30c through to the weekend while the west suburbs could top 40C with Penrith forecast to reach 45C on Friday.

Records have already started breaking with Borrona Downs in the state's northwest recording the highest minimum temperature ever in NSW with 34.6C on Monday.

The heat combined with thunderstorms and increased winds will result in severe bushfire danger.