The Bureau of Meteorology has issued a severe thunderstorm warning for GIANT HAILSTONES, INTENSE RAINFALL and DAMAGING WINDS for parts of the Northern Rivers that include Nimbin, Ballina, Casino, Mullumbimby and Kyogle.

A moist and unstable airmass east of a surface trough is combining with an upper trough, increasing the risk of severe thunderstorms across eastern and northeastern NSW throughout the evening.

Severe thunderstorms are likely to produce large, possibly giant hailstones, damaging winds and heavy rainfall that may lead to flash flooding over the next several hours in the Northern Tablelands and parts of the Northern Rivers, North West Slopes and Plains and Central West Slopes and Plains districts. Locations which may be affected include Armidale, Tenterfield, Gunnedah, Moree, Narrabri, Byron Bay, Ballina, Casino and Kyogle.

Severe thunderstorms are likely to produce intense rainfall that may lead to dangerous and life-threatening flash flooding, damaging winds and large hailstones over the next several hours in parts of the Northern Rivers district. Locations which may be affected include Murwillumbah, Numinbah, Tyalgum, Mullumbimby and Nimbin.

Numinbah recorded 84 mm of rain in 1 hour to 5:09pm.

Tevan (Alstonville STP) recorded 50mm in the 30 mins to 5:15pm.

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 8:30 pm.