Oppressive heat, elevated fire danger ratings, rain, thunderstorms and flooding are all on the forecast this Australia Day public holiday.

January 26 will see a hot air mass sitting over eastern Australia as a rain-bearing low pressure trough drifts across the nation's southeast. In the north of the country, rain and thunderstorms could cause flooding in some areas, while most of Western Australia will be dry.

The hottest capital cities on Tuesday will be Sydney and Canberra, where the mercury should reach around 35ºC. Sydney's western suburbs will be even hotter, likely reaching the low forties.

If Sydney does reach 35ºC in the city, which looks likely, it would be one of only five Australia Days to get this hot in records dating back to 1859.

Further south, a low pressure trough will cause rain and possibly a few thunderstorms in parts of Victoria, Tasmania, southern and western NSW and SA. Cooler air on the southern side of this trough will also bring some relief from the recent heat.

Adelaide hit 42.7ºC on Sunday but will only climb to about 24ºC on Australia Day, with a slight chance of showers.

Hobart should reach 21ºC on Tuesday, with some morning showers. This is considerably cooler than Monday's 37.5ºC.

Melbourne will also shrug off Monday's 39.2ºC with a top of just 22ºC forecast for Tuesday's public holiday. This cooler weather will be accompanied by some rain, particularly in the morning.

Brisbane and Perth should both escape the rain, with temperatures reaching the low thirties and high twenties respectively.

Darwin won't dodge the rain though, with widespread showers and thunderstorms likely over the nation's northern tropics. Some of this rain is likely to be heavy, especially over the western Top End and Queensland's Cape York Peninsula.