Large chunks of greater Sydney and NSW's central west are set for damaging winds and large hailstones as part of a severe thunderstorm.
The Bureau of Meteorology on Monday afternoon issued a severe thunderstorm warning for Sydney and parts of the Illawarra, Hunter and the Central Tablelands which could linger for most of the afternoon.
The bureau later extended the storm-prone area to NSW's Central West Slopes and Plains, as well as parts of the Southern Tablelands.
The State Emergency Service advised that cars in affected areas should be moved away from trees and loose items should be secured. People should stay at least eight metres from fallen power lines.
A severe thunderstorm cell was on Monday afternoon also moving in a northeasterly direction from western Sydney to the NSW Central Coast.
Meteorologist Helen Reid said earlier on Monday that storms had already been recorded across NSW's north and east, with gusts reaching 80km/h.
Some rain-hit areas could receive up to 30mm of rain on Monday.
"We do have a beautiful summer situation where we have a lot of moisture feeding in from the tropics," Ms Reid said in a statement.
"The tropical convergence zone has moved into the southern hemisphere, bringing that moisture closer to us, and with an inland trough over NSW just directing that moisture over us, it's going to hang around for a few days."
Most of coastal NSW is also subject to a strong marine wind warning.
Ms Reid said more thunderstorms would occur on Tuesday in northeast NSW, but a southerly change would ease conditions in NSW's south and west.