Adelaide is having its hottest Christmas Day in decades and has become the hottest capital city in Australia.

The Bureau of Meteorology says the temperature in the South Australian city reached 41.3C (106.3F) at 3.50pm.

"The last time when we had about 40C on Christmas Day was in 1945 and the temperature back then was 40.1C, so we've broken that record," forecaster Kenneth Cheung told AAP.

Image: A very hot Christmas on Glenelg Beach in Adelaide. Pic: Caroline Altenbernd

Beach goers - in Adelaide and other parts of the country - posted pictures of themselves sunbathing and trying to cool off in the water as a heatwave affected large parts of southern and eastern Australia.

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Melbourne recorded a maximum temperature of 36.3C, which was the hottest Christmas since 1998, according to local news reports.

It's just cracked 40°C in #Adelaide, the hottest Xmas city in the world! 🎄 🌞 🎅 #HoHoHot #ChristmasRoast — BOM South Australia (@BOM_SA) December 25, 2016

In Sydney, with a comparatively mild 27.4C, many crowded the popular Bondi Beach outside Australia's largest city.

Earlier this month, Sydney sweltered through its hottest December night in 148 years: 27.1C on 14 December.