The thunderstorms are likely to produce large hailstones, damaging winds and heavy rainfall that could lead to flash flooding in the next several hours.

Tamworth, Gunnedah and Armidale are expected to be affected and BOM meteorologist Gabrielle Woodhouse said the thunderstorms are likely to intensify in northern NSW through the afternoon and into tonight.

"These storms can produce damaging winds, a heavy burst of rainfall and damaging hailstones as well," she said.

"We are expecting those storms to be more widespread with severe thunderstorms in amongst that.

"Pretty much most of the state can expect some thunderstorm activity ... it's more likely we will see thunderstorms toward the northern inland."

The State Emergency Service (SES) has warned that trees damaged by bushfires are more likely to fall, and residents should keep clear of creeks and storm drains and avoid flood waters.

"Be aware that run-off from rainfall in fire affected areas may behave differently and be more rapid," an SES statement said.

"After bushfires, heavy rain and the loss of foliage can make the ground soft and heavy, leading to a greater change of landslides.

"Stay vigilant and monitor conditions, note that the landscape may have changed following bushfires."

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Residents should stay inside away from windows and keep pets indoors as well.

Vehicles should be moved under cover or away from trees.

It's advised computers and appliances should be switched off and people should avoid using the phone during the storm.

Any loose items around the home should be secured, and residents should keep at least eight metres away from fallen power lines or electric fences.

For emergency help call the SES on 132 500 or emergency services on 000.

A further update will be provided at 4pm.