The storm is predicted to deliver large hailstones, heavy rainfall and damaging winds.

The warning was issued at 12.26pm and covers people in the Northern Rivers, Mid North Coast, Hunter, Metropolitan, Illawarra, Northern Tablelands and parts of South Coast, Central Tablelands, Southern Tablelands and North West Slopes and Plains Forecast Districts.

Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding and damaging winds over the next several hours in the Hunter, Metropolitan, Illawarra and parts of the Mid North Coast, South Coast, Central Tablelands and Southern Tablelands districts. Locations which may be affected include Newcastle, Gosford, Sydney, Wollongong, Nowra and Katoomba.

Severe thunderstorms are likely to produce large hailstones and damaging winds over the next several hours in the Northern Rivers, Mid North Coast, Northern Tablelands and parts of the Hunter and North West Slopes and Plains districts.

Locations which may be affected include Lismore, Grafton, Coffs Harbour, Port Macquarie, Taree, Armidale and Tamworth.

West Pennant Hills recorded 72.5mm in 1 hour to 6:30am

Chatswood Bowling Club recorded 44mm in 30 minutes to 6:55am

Mossman recorded 61mm in 30 minutes to 7:30am

Box Hill recorded 40mm of rain in 30 minutes to 6am

The 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 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. 

* Keep clear of creeks and storm drains. 

* Don't walk, ride your bike or drive through flood water. 

* If you are trapped by flash flooding, seek refuge in the highest available place and ring 000 if you need rescue. 

* 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 3:30 pm.

If severe thunderstorms develop in Sydney, Newcastle, Wollongong or surrounding areas, a more detailed Severe Thunderstorm Warning will be issued to people in this area.

Warnings are also available through TV and Radio broadcasts, the Bureau's website at www.bom.gov.au or call 1300 659 218. The Bureau and State Emergency Service would appreciate warnings being broadcast regularly.