Air traffic is already being affected, with a number of flights cancelled. Sydney Airport said flights could be delayed 60-90 minutes through the day, and passengers are advised to check with their carriers.

Emergency crews were also out battling the winds to restore electricity to hundreds of homes and businesses. Areas affected range include Bundeena and Oyster Bay to Sydney's south.

The Bureau of Meteorology has issued a severe weather warning for an area stretching from the north of Sydney down to the Victorian border, and for a region between Scone and Taree. (See warning chart below.)

"The gusts will bring down trees and large branches, and have already done so," Brett Dutschke, a senior Weatherzone meteorologist, said.

The winds should strengthen during the morning and reach a peak in excess of 90km/h by between 10am and 2pm on Friday, he said.

Sustained "vigorous" winds will be about 60-70km/h, the bureau said. A bureau warning for hazardous surf has also been issued for the entire coast for Friday, with similar conditions expected on Saturday.

"It's going to be windy for the next couple of days," Mr Dutschke said, adding that the conditions on Friday were likely to be more severe than on the weekend.

The source of the winds is a strong low pressure system that is now in the southern Tasman Sea. It has generated a couple of strong cold fronts across southern Australian in recent days.

Gusts of as much as 87km/h were reported at Mount Boyce in the Blue Mountains at 12.44am on Friday.

These have since been exceeded with 98 km/h at Bellambi just before 9.30 am. Sydney Airport, meanwhile, has had gusts of 83 km/h at 10.44am, while nearby Kurnell has had 96 km/h winds.

Flights are already being affected and passengers are being encouraged to check with airlines.

Ausgrid was another firm taking to social media to alert customers of weather impacts.

For now, many of the homes affected by power cuts are located in areas south of Sydney:

Winds are expected to ease for most areas by Saturday, though vigorous winds will persist along the coastal fringe until Saturday night, the bureau said.

Blizzards are also likely over the Alps above 1400 metres, the bureau said. "The National Parks Service of NSW recommends that people consider postponing back country travel until conditions improve," it said in a statement.

Locations that may be affected include Sydney, Wollongong, Nowra, Batemans Bay, Goulburn, Katoomba and Cooma.

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