The Bureau of Meteorology has issued a Severe Thunderstorm Warning for LARGE, POSSIBLY GIANT HAILSTONES and DAMAGING WINDS for Lismore and most of the Northern Rivers.

A warm and very unstable airmass is in place over northeastern NSW. A trough moving north through the Northern rivers area combined with very strong winds aloft ahead of a potent upper trough system will provide an environment supportive of scattered severe thunderstorms today.

VERY DANGEROUS THUNDERSTORMS are likely to produce large, possibly giant hailstones and damaging winds in the warning area over the next several hours. Locations that may be affected include Lismore, Grafton, Byron Bay, Ballina, Casino, Kyogle, Evans Head, Yamba and Maclean.

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.

* 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 3:25 pm.