Wave of SES calls as thunderstorms hit NSW

"We're seeing damaging winds from the Victorian border all the way up through the Hunter area for many of our eastern elevated parts of NSW," Mr Narramore said.

Peak gusts of 100km/h are expected over numerous NSW towns today including Nowra, Bowral, Katoomba, Goulburn, Cooma and Bombala, with the Bureau warning winds will be strong enough to down trees, powerlines, and cause damage to property.

Severe, fast-moving thunderstorms are also currently tracking across northeastern NSW, with regional towns Coffs Harbour, Armidale, Glen Innes, Inverell, Sawtell and Dorrigo subject to severe weather warnings on Monday night.

Earlier, storm cells caused significant damage in Sydney and multiple members of the NSW State emergency Service were continuing to respond to calls Monday afternoon, NSW SES spokesperson Sean Kearns said.

Some of the damage included fallen trees and roofs being ripped off homes, he said.

"One of these in particular was at Mortdale where a large part of a workshop roof was removed due to the damaging winds," he said. "Thankfully no one was injured."

More than 270 calls for help came in to the NSW State Emergency Service on Monday between 6am and 2pm, with 185 from the Sydney metro area.

Trees were at higher risk of falling in high winds, after months of heavy rainfall softened soils, Mr Kearns added.

"People in the impact locations should move cars undercover and away from trees and powerlines and secure loose items around your home."

In the coming days, much of southeastern Australia will be plunged into an unseasonal cold snap, as a cold front brings showers, localised hail and severe thunderstorms to parts of NSW, Victoria, and Tasmania.

Temperatures could dive to between five and fifteen degrees below average for several days, with the frostiest conditions set to be on Thursday and Friday.

"For some, it could be the coldest December day or December night on record," Mr Narramore said.