Fire threat high despite cool relief as states swelter

Extreme fire danger warnings are in place with high bushfire risks across Australia's southeast. (Diego Fedele/AAP PHOTOS)

Relief is slowly arriving for some sunbaked Australians after several states sweated through one of the hottest December days in years.

Parts of Victoria, NSW, Queensland and South Australia topped 45C on Monday, while the Northern Territory faces severe to extreme heatwave conditions for much of the next three days.