Wild weather lashes parts of NSW

Sydney has been lashed by another afternoon of wild thunderstorms, as hail, flash floods and heavy rainfall brought down trees, damaged roofs and threatened properties.

The NSW State Emergency Service (SES) responded to more than 100 calls for assistance as storms began travelling through Sydney and surrounding areas on Sunday afternoon.

"The storms were a little bit shallower than Saturday, so they moved very quickly," SES spokesman Dave Owens told AAP.

"Because of the speed of the water, it went down as quickly as it came up."

But, he said, the storms still managed to wreak some mayhem as they swept through, damaging roofs, bringing down trees and causing flash flooding.

Two people were rescued from their home in the western Sydney suburb of Merrylands after water began inundating their property.

People in Campbelltown, Fairfield, Liverpool, Penrith, Blacktown and areas in the Blue Mountains were also affected, with reports of pea-size hail recorded in The Hills area, the SES said.

Lightning strikes damaged signalling equipment on the western and Blue Mountains lines, forcing trains to be stopped in both directions between Blacktown and Penrith on Sunday afternoon.

On the roads the storms also created traffic woes, causing numerous sets of traffic lights to black out across Sydney.

Motorists were advised to take extreme care, with traffic signals at Seven Hills, Bella Vista, Glenwood, Prestons, Bass Hill, Auburn, Ryde and Epping still out at 6.15pm (AEDT) on Sunday.

Meanwhile, a severe thunderstorm warning is current for many parts of the state including Lismore, Grafton, Penrith, Parramatta, Nowra, Bowral, Campbelltown, Braidwood and Bega.

The SES is closely monitoring the situation and has additional volunteers on standby to assist, it said.

More wild weather is predicted for Monday, with the Bureau of Meteorology (BoM) warning of isolated thunderstorms in the afternoon and early evening for Sydney.

Isolated thunderstorms are also expected to hit the Hunter, the Central Tablelands, the Illawarra, the Southern Tablelands, the south coast and the mid north coast.

Sunday's stormy weather came after large hailstones, heavy rainfall and flash flooding travelled through NSW on Saturday afternoon.

The State Emergency Service (SES) received over 200 calls for assistance on Saturday afternoon, including one from a Blue Mountains family trapped in their home.