Large swathes of the country are set to shiver through freezing conditions this weekend, with morning frost forecast across nearly every state and territory.
The frigid weather comes as a pool of cold air sweeps over the country, dragged up from the southern ocean by a series of cold fronts which whipped across southern Australia this week.
The Bureau of Meteorology (BOM) has forecast temperature to fall well below freezing through parts of inland New South Wales, WA, and the ACT on both Saturday and Sunday.
The nation's capital, Canberra, was expecting a low of minus 4 on Saturday morning, according to BOM.
Senior meteorologist Angus Hines said the cold snap was expected to continue for several days under clear skies.
"Following on from cold fronts up and across the south-east of the country on Friday, we've got really chilly air spreading out around the country … which is just going to get repeated on Sunday morning," he said.
Among the worst hit areas are eastern New South Wales away from the coastal fringe, the ACT, southern Queensland, central Australia and inland south-west WA.
Mr Hines said the chilly weather would last until mid-next week.
"Unfortunately it's not like we're going to flip a switch on Monday, it's still going to be clear and cold on Monday into Tuesday," he said.
"Large parts of the country will start to warm though, Tuesday and Wednesday, as we sort of get that next pulse of the warmer northerly wind setting up across parts of Australia."
Frost trends changing
Frost is a common occurrence this time of year for southern and central Australia, generally setting in during late autumn and finishing in early spring.
The latest Intergovernmental Panel on Climate Change report projected, with high confidence, that frost events would decrease, in general, across southern Australia in the future with climate change.
But other research has shown, at a more local scale, the downward trend of frost events hasn't been so clear.
Some parts of the country have even seen an increase in the number of annual frost days, according to data from the Australian National University (ANU), while neighbouring regions have decreased.
ANU climate applications scientist Steven Crimp said some parts of New South Wales were now experiencing five more frost events on average each year, compared to 1960.
He said this was based on local weather station data between 1960 and 2018, but the trend was unlikely to have changed much in the past five years.
"I think this is one of those climate surprises," he said.
"Despite the sort of overall warming trend in our temperatures, the extremes of our temperatures, be they hot or cold, are acting in a slightly more nuanced and complex way, which can be quite surprising at times," he said.
Dr Crimp said they had also found the frost season was lengthening across southern Australia.
"So if we think about the east coast first, we see an earlier start and a later finish to that frost window," he said.
"In some cases, the extension of that frost window is greater than 40 days.
"But in Western Australia in particular, we see that it's less to do with the later frost occurrence, but more earlier frost occurrence."
Why aren't frost days decreasing?
Dr Crimp said, ironically, the observations could be explained by the types of weather system that brought warmer, drier weather.
That was high pressure systems which often produced the clear, still nights needed for frost to settle.
"As anyone knows who's outside at night in winter, you have to have those clear night skies and the atmosphere needs to be very dry," he said.
"That way the surface of the Earth loses heat very rapidly and any moisture in the air then condenses as a frost.
"So because we are getting those dry conditions that are starting to emerge, that is more conducive for frosts to occur."
