After a relatively mild start to autumn, frosty weather arrived on the weekend courtesy of a low-pressure system off the NSW south coast that's expected to continue this week.
The Bureau of Meteorology warned of damaging winds affecting wide swathes of South Australia, Tasmania, Victoria, NSW and Queensland.
Widespread snow blanketed the alpine regions of NSW and Victoria including Kosciuszko National Park and Victoria's Falls Creek, with up to 20 centimetres in some areas, senior meteorologist Dean Narramore told AAP.
Snow was also falling across central western NSW in the Mudgee region and just outside Orange as well as Windeyer, Bocoble, Hargraves, Ilford, Oberon, Blayney, Carcoar and Lithgow.
By the afternoon, a wintry white hue was blanketing the hills in Canberra with the suburbs only a few degrees above zero, according to ACT Weather Watch.
The nearby town of Cooma notched up its coldest May day on record, he said.
In Sydney temperatures dropped to 11 degrees on Monday with winds averaging 60 to 70 km/h, with peak gusts around 90 km/h over the Illawarra and southern Sydney metropolitan coast.
Melbourne also had a brisk start to the day with temperatures registering just 8.5 degrees.
"Much colder southwesterly winds brought temperatures well below average for much of southeastern Australia," Mr Narramore said.
Sydney Airport experienced delays after just one runway was in operation for most of the morning due to strong wind gusts, forcing dozens of domestic flights to be cancelled.
Heavy surf lashed beaches as storm-force winds swept along the coast, with many closed to swimmers as waves up to six metres smashed onto shore.
A hazardous surf warning is in place for much of the NSW coast with warnings it could cause damage and coastal erosion between the Victorian border and Seal Rocks, with conditions to ease along the coast south of Batemans Bay on Monday night.
Spring high tides could compound the impact of waves bringing hazardous surf conditions along the coast.
"Beach conditions in these areas could be dangerous and people should stay well away from the surf and surf exposed areas," the BOM said.
A separate severe weather warning was issued for damaging surf particularly for Kurnell, Wollongong, Bulli, Port Kembla, Albion Park, Kiama, Jervis Bay and Huskisson.
On the NSW south coast, a 113 km/h wind gust was recorded at Barunguba on Montague Island and a 98 km/h gust at Green Cape lighthouse near Eden early in the morning.
Marine Rescue NSW advised boaters to stay off the water in areas where warnings were issued.
NSW sheep graziers were warned that frosty temperatures and southwesterly winds could affect livestock inland.
"There is a risk of losses of lambs and sheep exposed to these conditions," the BOM said.
The cold air is expected to linger for the rest of the day and into Tuesday, though heavy rains and snow had subsided in most regions by Monday afternoon.
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