Canberra and Adelaide just had their coldest morning of 2018 after temperatures plummeted across southern Australia during the longest night of the year.

A broad ridge of high pressure across southern Australia caused clear skies and light winds during Thursday night, allowing temperatures to drop more than five degrees below average in some areas.

Shortly after the sun rose on Friday morning, Canberra had reached -5.1 degrees and Adelaide 3.5 degrees, making this the coldest start so for this year in both cities.

While the high pressure ridge draped across southern Australia played a big part in this morning's chilly minimum temperatures, there was also more cooling time. Thursday night was the longest night of 2018 across Australia because the southern hemisphere's winter solstice occurred at 8:07pm Eastern Standard Time on Thursday 21st June.

After a long night, Mildura's -2.1 degrees on Friday morning was seven below average and its lowest June temperature in six years.

Thursday night will be another cold and frosty one for much of central and southern Australia, although there will be a little bit less darkness across the country compared to last night.