The severe weather warning for large hailstones has been extended to Lismore, Ballina and Evans Head.

The Bureau of Meteorology says isolated thunderstorms have developed underneath an upper trough over northeastern parts of the state. Storms are slowly moving to the northeast and may persist into the early evening.

Severe thunderstorms are likely to produce large hailstones in the warning area over the next several hours. Locations that may be affected include Lismore, Byron Bay, Ballina, Casino, Kyogle and Evans Head.

3 cm hail was observed near Tabulam at 4:40 pm.

The State Emergency Service advises that people should:

* Move your car under cover.

* 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 8:25 pm.