Firefighters are on high alert with parts of Australia tipped to have seven days above 40C.

Roxby Downs and Oodnadatta in South Australia's outback are set to sizzle with six days in a row above 45C and the tiny town of Tarcoola is forecast to reach 47C on Monday.

Temperatures will remain above 40C for the next six or seven days in many other inland parts of Australia including the towns of Mildura and Ouyen in Victoria.

Victoria is set to experience its longest stretch of heat and some of its worst fire conditions since Black Saturday when 173 people died.

Many towns in eastern Western Australia and western Queensland will also feel the heat, with temperatures set to soar into the 40s in parts of both states.