The Bureau of Meteorology has issued a Severe Thunderstorm Warning for Large Hailstones, Damaging Winds and Heavy Rainfall for people in Casino and the western parts of the Northern Rivers.

A front and associated trough is crossing through central parts of the state, reaching the northeast during the afternoon. Severe thunderstorms have developed in the moist and unstable airmass ahead of the trough.

Storms are moving to the east and northeast and are likely to continue throughout the afternoon and into the early evening.

Severe thunderstorms are likely to produce large hailstones, damaging winds and heavy rainfall that may lead to flash flooding in the warning area over the next several hours. Locations that may be affected include Casino, Grafton, Kempsey, Armidale, Dorrigo and Macksville.

At this stage, the warning does not include Lismore and the coastal areas of the Northern Rivers.

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 (131 388), Endeavour Energy (131 003), Essential Energy (132 080) or Evoenergy (131 093) as shown on your power bill.

* Trees that have been damaged by fire are likely to be more unstable and more likely to fall.

* 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.

* Be aware that run-off from rainfall in fire affected areas may behave differently and be more rapid. It may also contain debris such as ash, soil, trees and rocks.

* After bushfires, heavy rain and the loss of foliage can make the ground soft and heavy, leading to a greater chance of landslides.

* Unplug computers and appliances.

* Avoid using the phone during the storm.

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

* Stay vigilant and monitor conditions. Note that the landscape may have changed following bushfires.

* 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 7:10 pm.