Gale-force winds have torn the roof off a nursing home near Newcastle, as dangerous weather continues to wreak havoc across parts of NSW.

Key points: Only one runway is in operation at Sydney Airport, causing average delays of 45 minutes

Only one runway is in operation at Sydney Airport, causing average delays of 45 minutes The Bureau of Meteorology is warning the "worst of the cold air" will reach NSW on Saturday

The Bureau of Meteorology is warning the "worst of the cold air" will reach NSW on Saturday It's also warned of blizzard conditions in alpine areas with the risk of avalanches

Windspeeds of up to 98 kilometres per hour were recorded at the Central Coast as a deep polar vortex moves across south-east Australia — including parts of Victoria and South Australia — causing alpine blizzards, delays and cancellations at Sydney airport and traffic delays.

Two out-of-control bushfires on the state's Mid North coast which were being fanned by strong winds have been downgraded, the Rural Fire Service said.

But they are warning the situation could worsen tomorrow when wind gusts are forecast to pick up again.

Emergency services were called around 10:00am to Wescott Presbyterian aged care centre in Stockton, to find an extended section of the building's roof lying on the street.

Around 30 residents were in the damaged part of the building, but NSW Ambulance said no-one was injured.

NSW State Emergency Services and NSW Fire and Rescue are on the scene removing debris and alternative accommodation is being sought for affected residents.

The section of roof was covering the rooms of 30 residents at the nursing home. ( ABC News: Ben Millington )

Almost 200 domestic flights at Sydney Airport have been cancelled, with only one runway in operation, causing an average 90-minute delay to domestic flights.

Some international flights may be delayed by up to half an hour.

Domestic and international travellers are being urged to check directly with their airlines to see if their flights are affected.

Passengers expressed dismay at having their travel plans ruined.

Nora Murphy planned to fly to Brisbane for the weekend to see her daughter, but received a text message from Tigerair to say her flight was cancelled.

"I have now paid $800 for another flight, which is now delayed again," she said.

Another woman with three children has been trying to get to Brisbane to catch a connecting flight to Fiji for a family holiday.

"They've tried rebooking our flights and then they've cancelled all three bookings," she said.

Watch and Act alerts have been downgraded for fires at Turners Flat and Dondingalong, near Kempsey.

James Morris from the Rural Fire Service said winds near the fires were gusting up to 60 kph from the north west which fanned the flames.

The NSW Rural Fire Service is battling out-of-control bushfires on the state's Mid North Coast. ( ABC News: Luisa Rubbo )

The Bureau of Meteorology (BOM) warned the "worst of the cold air" was expected to reach NSW on Saturday.

"This is the coldest outbreak the south-east has seen this year," BOM duty forecaster Rohan Smyth said.

Most of the state's east coast was issued a damaging wind warning for more than 90 kph on Friday.

A blizzard warning for Victoria and NSW's alpine areas remained in place, with winds peaking at 118 kph just before 4:00am.

"This dump of snow will increase the risk of avalanche, so skiers outside of resort areas should assess the avalanche risk," Mr Smyth said.



Just before 5:30am, the apparent temperature at Thredbo top station was -22 degrees Celsius.

The NSW National Parks and Wildlife Service recommended backcountry travel be postponed until conditions improve.

The State Emergency services said it had receive more than 400 requests for help since midnight, mostly for fallen trees, fallen power lines and roof damage.

There were 166 calls from Sydney, 35 from the Blue Mountains and 56 from Wollongong.