Temperatures dropped well below zero overnight in Victoria, Tasmania, New South Wales and the ACT, just days after one of southeastern Australia’s most intense heatwaves in recorded history.

Even several South Australian towns plummeted towards zero overnight, after the mercury soared to 50°C at two SA locations late last week. The lowest temperature recorded in SA on Monday morning was 0.8°C at Coonawarra in the state's east.

In the four states where temperatures fell below zero on Monday morning, the lowest recorded temperatures were:

VIC: Mt Hotham -3.7°C

NSW: Perisher -2.1°C

ACT: Mt Ginini -2.0°C

TAS: kunanyi/Mt Wellington -1.0°C

What caused the unseasonable chill?

A polar airmass pushed into Tasmania during Sunday, delivering showers and highland snowfalls to our southernmost state.

The airmass continued to surge northwards but by the time it reached the southeast mainland on Sunday afternoon and evening, it had largely dried out.

While the loop above shows rain and storm activity in the warm northwesterly flow ahead of the cold front, it also illustrates that very little moisture accompanied the cool change on the mainland.

The rapidly drying airmass is what helped temperatures drop so low overnight. And it could have been even colder if winds had not remained strong and gusty across the southeast, preventing further radiational cooling in valley locations.

A run of clear days with cool nights should set in for the first half of the week across most of the areas mentioned. Tuesday morning will again be relatively cool by summer standards, but not as cold as Monday morning.