A severe to extreme heatwave will touch part of every mainland state and territory this week, raising fire danger ratings and challenging records.

A large mass of hot air will spread across Australia from west to east between now and the weekend. Temperatures are forecast to reach the mid-to-high forties in parts of every mainland state and the NT during the next several days, with some places likely to near 50 degrees.

Image: Heatwave forecast for Wednesday to Friday. Source: Bureau of Meteorology.

The hottest air will lie over central and southern Australia on Wednesday and Thursday, before spreading over southeastern Australia from Friday and pushing up into NSW and southern Queensland on the weekend.

While numerous December heat records and some all-time records could be broken this week, the highest temperatures are likely to occur on Thursday somewhere in SA or WA, between about Forrest and Wudinna. This stretch of land is sparsely populated by people and weather stations, so the highest temperature of the day may occur without being registered.

Image: Forecast surface temperature on Thursday, December 19th according to the ECMWF-HRES model.

If any weather station reaches 50 degrees this week, it will be the first time this has officially been recorded in Australia during December. The mercury would need to reach closer to 51 degrees to beat the all-time national record of 50.7 degrees from Oodnadatta in January, 1960.

Image: Australia's national and state heat records prior to this week, according to the Bureau of Meteorology's online climate database.

In addition to the health risks associated with this week's heatwave, the burst of hot weather will also elevate fire danger ratings across southern and eastern Australia.

Fire danger ratings will rise in South Australia from Wednesday and could reach Severe to Catastrophic levels across the entire state on Friday. Severe or Extreme fire danger ratings are also likely in parts of WA, Victoria, NSW, the ACT, Tasmania and Queensland during the second half of this week.