Severe storms have hit Sutherland, Campbelltown, Liverpool, Picton, Camden, and a severe storm cell is moving towards Wollongong, Bulli and Thirroul, Weatherzone meteorologist Rob Sharpe said.

''The [Sydney] CBD will get the storms but a severe cell is moving through the south of Sydney and into the Illawarra,'' he said.

Mr Sharpe said the severe storm warning covered much of northern and eastern New South Wales.

"This is one of the biggest storm days we’ve seen this year," he said.

"There’s a long [storm] band running from the Queensland border up near Moree, all the way down through to the Illawarra, pretty much a line of storm, and that’s moving east across the state."

Areas under threat from severe storms include Gosford, Newcastle, Goulbourn, Mudgee, Orange, Scone, Taree, Tamworth, Armidale, Dubbo, Katoomba, Nowra, and Port Macquarie.

Large hailstones were reported this afternoon during storms at Orange, Mount Victoria and Nowra, while flood warnings are in place for the Hunter and Manning River valleys.

Motorists are warned to exercise caution, with fallen trees and localised flooding slowing traffic on some roads.

In Sydney, a fallen tree closed two northbound and one southbound lane of the Pacific Highway at Campbell Street in Artarmon about 3pm. There were also heavy traffic conditions on the Anzac Bridge approaching the City-West Link in Rozelle, with westbound traffic affected. Flooding was also affecting Epping Road at Culloden Road in Marsfield.

Mr Sharpe said thunder and showers were forecast to continue across Sydney into this evening.

"Tomorrow’s going to see a clearing trend, there might be some storms and showers easing but they’ll be easing off by the afternoon," he said.

"We’ll see some sun in the afternoon. But it will stay quite cool, 22 degrees for Sydney and fresh and gusty winds."