Many places over Australia's interior will experience a run of heat only experienced once-or-twice before in the last 50 years, according to WeatherZone.

The area includes Eucla, Tarcoola, Oodnadatta, Marree, Moomba, Birdsville, Broken Hill, Hay and Mildura.

Much of central Australia, including Tarcoola, Coober Pedy, Roxby Downs, Oodnadatta, Marree, Moomba, Birdsville and Uluru, should average a maximum of about 45 degrees for at least a week and 42-45 degrees for a fortnight.

Birdsville is on target to reach at least 40 degrees each day for three weeks. Broken Hill, Hay and Mildura are a fair chance to get five consecutive days of 42 degrees or hotter.

This sort of heat has only occurred once in the least 50 years in Broken Hill and Mildura and twice in the last 50 years in Hay and Birdsville.

For most of the interior, the last time there was a longer hotter spell was surrounding Black Saturday in February 2009.

For coastal areas in southeastern Australia, including Adelaide, Melbourne, Hobart, Sydney and Brisbane weak cool changes will be enough to interrupt the extreme heat after one or two days. However, hot weather will return a few days later, although it is unlikely to be as extreme as the first round.

For Adelaide, Melbourne and Hobart, this Friday will be their hottest day as dry northerly winds develop. Adelaide and Melbourne will reach the low 40s and Hobart the high 30s, about 15-or-16 degrees above average. Melbourne typically reaches 40 degrees only once or twice each summer and Hobart has not reached 38 in January since 2006.

The dry northerlies affecting these cities will be gusty enough to increase the bushfire risk across a large area of southeastern Australia, to levels not experienced since Black Saturday.

A weak front will bring a cooler change to Adelaide on Friday afternoon and to Hobart and Melbourne later that night.

The first round of heat will take longer to reach the coast of New South Wales and Queensland. Sydney and Brisbane are only likely to experience a brief burst of temperatures in the mid 30s-to-low 40s next Tuesday and Wednesday respectively.

The origin of this heat is the WA Pilbara. In the lead-up to Christmas the nearby seas warmed up to be three-or-four degrees above average with help from a long run of sunny days. This warmer-than-normal air allowed inland areas to heat up substantially with Nyang reaching 47 two days before Christmas. During the last week of 2012 northerly winds dragged this heat to southern WA, where Perth experienced its hottest week in 35 years and hottest December week on record, averaging a maximum of 38 degrees.

Since the start of 2013 cold fronts have cooled southwest WA and have pushed the very hot northerly winds further east.

The hottest place so far has been Eucla, where it reached 48.1 degrees this afternoon, its hottest day on record and 22 degrees above the summer average. Yesterday nearby Eyre had its hottest day in at least 30 years, reaching 47.7, 21 above average.