The Bureau of Meteorology warns that, at 1:35 pm, severe thunderstorms were detected on the weather radar near Calga, Spencer, Upper Mangrove, Mangrove Mountain, Mangrove Creek Dam, the Wollemi National Park southwest of Singleton and Broke.
These thunderstorms are moving towards the east.
They are forecast to affect Gosford, Wyong, Maitland, Cessnock, Branxton and Kurri Kurri by 2:05 pm and Newcastle City, Raymond Terrace, Charlestown, Wallsend, Medowie and Tocal by 2:35 pm.
Large hailstones and damaging winds are likely.
The State Emergency Service 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 8 metres away from fallen power lines or objects that may be energised, such as fences.
* Report fallen power lines to either Ausgrid on 131 388, or Endeavour Energy on 131 003 or Essential Energy on 132 080, as shown on your power bill.
* Unplug computers and appliances.
* Avoid using the phone during the storm.
* Stay indoors away from windows, and keep children and pets indoors as well.
* For emergency help in floods and storms, ring the SES (NSW and ACT) on 132 500.
The next warning is due to be issued by 2:35 pm.