It feels like spring hasn't started in parts of southeastern Australia, with some places registering their lowest September temperatures in years during the last couple of mornings.

A cold air mass combined with clear skies and light winds is producing unusually cold mornings in parts of South Australia, Victoria, NSW and the ACT this week.

Despite being the first month of spring, Victoria's Nhill Airport registered its lowest temperature in three years on Tuesday morning, dropping to minus 1.2 degrees just before 6am. This was also the coldest September night in more than 15 years.

Elsewhere, South Australia's Hindmarsh (4.3C) experienced its coldest morning this late in the year for 15 years on Tuesday, while Coldstream (-0.2C) in Victoria and Sheffield (-0.5C) in Tasmania had their coldest mornings this late in the year since 2010.

On Monday, Australia's highest weather station, Thredbo Top Station, dropped to a numbing minus 9.7 degrees. This was Australia's lowest September temperature in five years. Mount Hotham's minus eight on 16th September was also Victoria's lowest temperature for the month since 2013.

The lack of rain in recent months is likely to have contributed to this late-season chill. Overnight cooling is enhanced when the air is dry and skies are clear.