The warning indicated that a low pressure system over inland New South Wales was drawing humid air across eastern parts of the state causing widespread thunderstorm activity.

At 1.45pm, the BOM warned that severe thunderstorms in the west of the state were moving south-east and are likely to affect Branxton by 2.45pm.

According to the BOM, severe thunderstorms are likely to produce damaging, locally destructive winds, large, possibly giant hailstones and heavy rainfall that may lead to flash flooding in the warning area over the coming hours.

In the event of severe storm weather, the State Emergency Service (SES) advises that people move cars from away from trees; secure loose items around the house, yard and balcony; and stay at least eight metres from fallen power lines or potentially energised objects like fences.

Residents can report fallen power lines to the energy provider shown on their power bill, including Ausgrid (131 388), Endeavour Energy (131 003), Essential Energy (132 080) or Evoenergy (131 093).

It is safest to stay away from creeks and storm drains and to refrain from walking, riding or driving through flood water.

For emergency help in NSW floods and storms, contact the SES on 132 500.