The Bureau of Meteorology has issued a Severe Thunderstorm Warning for HEAVY RAINFALL for Casino, Kyogle and other parts of Northern Rivers.

A very moist and unstable airmass supported by an approaching upper trough is producing scattered showers and isolated severe thunderstorms over the northeast of the state.

In the west, severe thunderstorms have developed and will continue to move to the east throughout the afternoon and into the early evening.

Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding in the warning area over the next several hours. Locations that may be affected include Casino, Kyogle, Tabulam, Baryulgil, Rappville and Whiporie.

At this stage, the Thunderstorm Warning misses Lismore and coastal towns.

89.0 mm was recorded at Tidbinbilla in the 6 hours to 3:00 pm.

63.8 mm was recorded at Gudgenby in the 6 hours to 2:30 pm.

94.2 mm was recorded at Mount Darragh in the 6 hours to 12:27 pm.

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 10:50 pm.