There is a total fire ban in place for most of South Australia, as well as fire restictions for all of Victoria as a result of the conditions.

Temperatures started soaring across South Australia yesterday with the Nullarbor reaching 38 degrees - 10 degrees above the January norm, Weatherzone is reporting on its FarmOnline weather service.

It says other sweaty locations yesterday included Ceduna and Wudinna, which topped at 37.

Today Coober Pedy and Leigh Creek were expected to reach 40, Adelaide, Clare and Keith 37, and Whyalla 38.

While conditions should ease somewhat on the coastal strip from Tuesday, inland areas like Clare, Renmark and Leigh Creek can expect temperatures of between 35 and 40 right through to the weekend.

It's a similar story in southern and western NSW, as well as Victoria.

Mildura was expecting a top of 39 today, with 37 forecast for Bendigo, Horsham and Wangaratta, and 36 for Seymour.

Temperatures will ease for centres closer to the Victorian coast on Wednesday, but inland centres can still expect daily highs of around 35 degrees through until Sunday.

All of Victoria can expect extreme heat on Saturday.

In NSW it will be the Riverina as well as centres like Broken Hill and Dubbo which will bear the brunt of the heatwave.

Temperatures between 34 and 38 will be the norm in those areas, but Weatherzone is forecasting thunderstorms for all parts of NSW for most days this week.