Flights have been diverted and delayed at Melbourne Airport as thunderstorms, hail and a tornado batter the city.
Melbourne airport spokeswoman Anna Gillett says lightning strikes in the area mean that planes have been grounded while the bad weather continues.
She recommended passengers heading out of Melbourne should contact their airline or the airport for more information.
Much of Melbourne, and particularly the north and north-west of the city, have been hit by huge hailstones.
State Emergency Service spokesman Lachlan Quick said the organisation had received 950 calls for help by 7pm.
He said 90% of the calls concerned smashed windows and skylights, while he believed the damage to cars from hailstones will be widespread and severe.
"There was a tornado reported at Fiskville and we know it touched down at Keilor Downs because we had some SES volunteers there and they had to run for shelter," Quick said.
Severe thunderstorm likely in southern NSW
Sydney's Christmas Day sunny weather looks likely to come to an end, with a severe thunderstorm warning issued across southern NSW.
People were being warned this afternoon to expect large hailstones, flash flooding and damaging winds, with locations that may be affected including Nowra, Batemans Bay, Goulburn, Canberra, Cooma, Yass, Wagga Wagga and Albury.
The severe weather warning has been issued for people in the southern tablelands, the south-west slopes, the Riverina, the Snowy Mountains, parts of the Illawarra and the south coast.
Parts of the the central tablelands, central west slopes and plains and upper western districts and the ACT are also likely to be affected.
For Sydneysiders, Christmas Day ushered in sunny skies and a maximum temperature of 26 degrees.
But possible showers are predicted for Boxing Day, with a 90% chance of rain.
In Sydney, scattered showers and isolated thunderstorms are expected to develop from midday, with northeasterly winds averaging up to 20km/h in the west and 30km/h near the coast.
Later in the evening a gusty southerly change is forecast, with winds reaching 40km/h near the coast.
The wet weather change is expected through much of the state on Christmas Day evening, with isolated showers expected on the mid-north coast, northern tablelands, central tablelands, the Hunter, parts of the Illawarra, central west slopes and plains and the Snowy Mountains.
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