Damaging winds, and possible hailstones, are expected, too, on November 26, according to the BoM alert at 12.20pm.

A cold front crossing the state is generating gusty thunderstorms.

However, it's expected to move out to sea later in the evening.

Locations, which may be affected, include Merriwa, Denman, Muswellbrook, Scone, Aberdeen, Mudgee, Dubbo, Newcastle, Gosford and Sydney.

The NSW State Emergency Service (SES) advises that people should:

* Move your car under cover or away from trees;

* Secure or put away loose items around your house, yard and balcony;

* Keep at least eight metres away from fallen power lines or objects that may be energised, such as fences;

* Report fallen power lines to either Ausgrid (131 388), Endeavour Energy (131 003), Essential Energy (132 080) or Evoenergy (131 093) as shown on your power bill;

* Unplug computers and appliances;

* Avoid using the phone during the storm; and

* Stay indoors away from windows, and keep children and pets indoors as well.

For emergency help, phone the SES (NSW) on 132 500.