The potential thunderstorms we were warned about over the past few days are eventuating for residents in Nimbin, Casino and Kyogle; however, not for Lismore or Ballina at this stage.

The Bureau of Meteorology has issued a Severe Thunderstorm Warning for LARGE, POSSIBLY GIANT HAILSTONES, DAMAGING WINDS and HEAVY RAINFALL.

The warning includes potential flash flooding.

A trough extends over northeastern NSW with a warm, humid and unstable airmass on its eastern side. Scattered thunderstorms are expected to develop east of this main trough during the afternoon and evening.

Moderately strong winds in the upper atmosphere will promote organised severe thunderstorms, eventually developing into lines of storms in the late afternoon and evening.

Residents in Urbenville (west of Kyogle) may be in for VERY DANGEROUS THUNDERSTORMS that are likely to produce large, possibly giant hailstones, damaging winds and heavy rainfall that may lead to flash flooding over the next several hours.

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.