The spring temperature see-saw swung wildly in southeastern Australia on the weekend, replacing dangerous fire weather with frost and snow.

A warm air mass passing over the nation's southeast on Saturday caused the mercury to climb more than 10 degrees above average in some areas.

Sydney Airport's top of 33 degrees on Saturday was 13 degrees warmer than usual for September and one of only three days since 1947 the site has been this hot this early in the season.

In Victoria, Mallacoota's 29C on Saturday was its highest temperature this early in the season for five years and nearly 12C above average for early spring.

While some people enjoyed this early taste of summer, the warm and windy conditions prompted total fire bans in parts of northern, central and eastern NSW for the first time this season.

However, the warmth was short-lived. Unusually hot days at this time of year are always a sign that cold air is on the way.

A strong cold front trailing the tongue of warm air caused the mercury to plummet as it marched from the Bight towards the Tasman Sea.

Ulladulla's maximum temperature dropped from 31 degrees on Saturday to just 15 degrees on Sunday and it was even cold enough for snow at some of Australia's ski resorts. Fresh flakes were seen falling at Mt Buller, Mt Baw Baw, Falls Creek, Thredbo and Perisher.

The cold air lingering over southeastern Australia is now combining with clearer skies and lighter winds underneath a high pressure system to cause cold and frosty mornings.

Monday morning was the coldest this late in the year for more than 60 years at Wilcannia (-0.7C) and Parkes (-3.3C), and in 50 years at Wagga Wagga (-2.6C).

Hot and cold air will continue to battle it out over southern Australia in the coming weeks until cold fronts contract further south during the summer months and warmer weather becomes more dominant.