The warning for thunderstorms we had written about earlier in the day are a reality with the Bureau of Meteorology announcing a Severe Thunderstorm Warning for Lismore and the Northern Rivers at 5:20pm today (Monday).

The Bureau explains the reason as a low in Northwest Victoria is extending a trough up into Southwest Queensland. Thunderstorms are developing in the moist airmass to the east of this trough today. Thunderstorms are also developing near the northeast coast.

Severe thunderstorms are likely to produce heavy rainfall that may lead to flash flooding, large hailstones and damaging winds in the warning area over the next several hours. Locations which may be affected include Lismore, Grafton, Orange, Dubbo, Cobar and Bourke.

Young recorded 21mm in the 30 minutes to 4:15pm.

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.