A major swath of central Australia stretching from Oodnadatta to Coober Pedy and Birdsville can expect maximum temperatures of 45 degrees or hotter for at least a week.

Towns to the south and east, such as Mildura and Hay, can expect to broil with 40-degree maximum temperatures for just as long.

''We have a major heat event under way,'' Karl Braganza, manager of climate monitoring at the Bureau of Meteorology, said. ''There are not many instances in the historical record where you get a heat event covering such a large area of the continent.''

Brett Dutschke, senior meteorologist at Weatherzone, said it was unusual to have so prolonged a hot spell. ''It's a once-in-20 or 30-year heatwave event in desert areas,'' he said. ''More populated areas further south … are going to experience some of this as well.''

The mercury is forecast to hit 36 degrees in Melbourne on Thursday and 41 degrees of Friday, with temperatures also soaring in Canberra although Sydney will largely be spared.

Much of south-eastern Australia has weathered warm temperatures for months. ''We effectively had midsummer heat arriving at the end of spring,'' Dr Dutschke said.

Nationwide, though, 2012 was probably an average year, temperature-wise, with the Bureau of Meteorology due to release figures Thursday morning.

The rainfall shift, though, was stark. Much of the continent went from cool and wet in the March quarter to drier-than-average conditions within a short spate of time.

"The stunning reversal in rainfall from wet to dry across large parts of the continent, particularly the south, is a result of a switch from a La Nina (weather pattern) early in the year to El Nino-like conditions through winter and spring,” Dr Braganza said.

While much of south-eastern Australia enters its high fire danger period, there may be some modest relief on the way. Weatherzone’s Dr Dutschke says models indicate temperatures and rainfall will start to trend back to more normal summer patterns from about the middle of January.