The Bureau of Meteorology warned that at 3pm, severe thunderstorms were detected on the weather radar near Maitland and Raymond Terrace. These thunderstorms are moving towards the east to southeast.

They are forecast to affect Nelson Bay and Karuah by 3.30 pm and waters off Port Stephens by 4pm. Large hailstones and damaging winds are likely.

Earlier:

The Bureau of Meteorology has detected storm activity in the Upper Hunter and has issued a severe weather warning as the system moves east toward Raymond Terrace and Medowie. The BoM issued warns that, at 1:35 pm, when 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.

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.