Heatwave peak still to come as temperatures reach high 40s

The mercury hit almost 49C in some parts of the country as Australia swelters through a week-long heatwave, but the worst is yet to come.

South Australians are trying to stay cool as the state faces a third day of soaring temperatures. (AAP)

Australia's scorching summer has continued as extreme heatwave conditions pushed temperatures in some regions to 80-year highs.

In NSW, much of the state was baking in high intensity heat on Wednesday but the worst is to come with temperatures to peak later in the week.

Record heat in South Australia on Tuesday moved into much of western and Central NSW on Wednesday, which saw temperatures in the high 40s across much of those areas, a Bureau spokesman told AAP.

The majority of the state is forecast to exceed 41C until Friday, something the Bureau says has not been seen in 80 years.

A mother and child cool off in the water. Source: Getty Images AsiaPac

Whitecliff in the state's northwest, saw the highest temperature in NSW with 48.2C just before 3.30pm.

By midday on Wednesday, the mercury had soared beyond 45C across much of NSW's central west and at 3pm Wilcannia, Mulurulu, Ivanhoe and Hay topped 47C.

In Sydney's west temperatures reached 39C in Penrith while at Observatory Hill in the CBD it was a cooler 30C in the afternoon.

Sydney train commuters are being warned there could be delays across the network as temperatures rise.

In South Australia, a record-breaking heatwave has seen 16 people with heat-related conditions presented at hospital emergency departments across the state in the last 24 hours, with seven admitted for further treatment.

A dog plays in a sprinkler at Queens Park in Sydney as temperatures surpass 40C in several parts of Australia. Source: AAP

More admissions were expected with health authorities warning health issues were more likely after several days of sustained hot weather.

Over the past three days maximum temperatures across South Australian have been running 10 to 14 degrees above average.

Some regional centres posted record highs on Tuesday, including Port Augusta and Tarcoola where the mercury climbed to 49C and 48.9C respectively, all-time highs for those areas.

Adelaide had a top of 40.2C in the city but temperatures were slightly higher in some outlying suburbs.

Young spectators seek relief from the baking heat at the Australian Open. Source: Getty

While in Melbourne's CBD the mercury only reached 26.7C, elsewhere in Victoria reached the mid 40s.

In Mallee, Swan Hill reached 45.9C just before 5pm - just 0.3 degrees away from the area's January record - while Mildura hit 45.4C.

In the north, Yarrawonga reached 45.7C and Mangalore reached 44.8C.

Extreme heatwave conditions were also forecast for parts of Queensland's south.

In Ballera and Birdsville the mercury reached 45.3C after 2pm and Windora was 43.3C just after 4pm.