The Bureau of Meteorology has issued a Severe Thunderstorm Warning for heavy rain in Casino but missing Lismore at this stage of the evening.

An upper trough and series of surface troughs are resulting in slow-moving thunderstorms this evening. As is the case with storms, they can be unpredictable.

The Bureau say, "Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding in the warning area over the next several hours. Locations which may be affected include Wellington, Casino, Tabulam, Maclean, Cowra and Tumut."

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 forecast is for 0-6mm to fall in Lismore for Tuesday.

The next warning is due to be issued by 9:10 pm.