Parts of Victoria and Tasmania registered their lowest September temperatures in several years this morning as widespread frost affected southeastern Australia.

Cool air lingering underneath a high pressure system caused temperatures to drop seven to eight degrees below average in parts of Australia's southeast on Thursday.

Victoria's Hopetoun dipped to minus 2.8 degrees early this morning, which was its lowest September temperature in seven years.

In Tasmania, Fingal had its coldest September morning in five years after reaching a low of minus 4.4 degrees shortly before 6:30am.

Melburnians awoke to a nippy start as the mercury dipped to 3.8 degrees. While there was a lower temperature recorded earlier this month, this was the city's coldest start this late in the year for 15 years.

Temperatures were also cold enough for frost this morning in parts of southern NSW, the ACT and SA's eastern inland. Canberra reached -2.9 degrees, which is the city's coldest start so far this spring.

Friday morning will be slightly warmer for most of southeastern Australia, although frost is likely to spread further north along the ranges of NSW.