The Bureau of Meteorology has issued a Severe Thunderstorm Warning for HEAVY RAINFALL for people in Nimbin and Kyogle and other parts of the Northern Rivers.

Severe thunderstorms are developing in a moist and unstable airmass over northern parts of the state. Storms are slow-moving and are likely to continue throughout the afternoon.

Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding over the next several hours in parts of the Northern Rivers and Northern Tablelands districts.

Locations which may be affected include Kyogle, Tabulam, Urbenville, Wiangaree, Nimbin and Mallanganee.

The State Emergency Service advises that people should:

* 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 6:10 pm.