Newcastle SES Unit Commander Ian Robinson said around 29 millimetres of rain drenched the Maitland area in just an hour, as severe winds brought down trees and broke windows.

Crews from Newcastle, Lake Macquarie and Wyong have responded to calls for help and Mr Robinson said the SES were likely to be active for at least the next two days dealing with the storm's aftermath.

Newcastle appeared to be spared the brunt of the weather, recording around seven millimetres of rain over the city since 9am.

Storms eased over the city around 4pm, however a severe weather warning remained in place for parts of the Hunter and Mid North Coast, as forecasters anticipated the possibility of further heavy rain and strong winds that could lead to flash flooding.

"The redevelopment of severe thunderstorms remains likely," the Bureau of Meteorology said in a statement at 3.35pm.

"The situation is being closely monitored and further detailed warnings will be issued as necessary."

Earlier the Bureau said afternoon storms are likely to deliver "damaging winds, large hailstones and heavy rainfall that may lead to flash flooding" across the Hunter on Monday afternoon.

Forecasters issued a warning shortly after 2pm warning Newcastle, Scone, Cessnock, Maitland, Armidale, Tamworth, Gunnedah, Moree and Narrabri to batten down the hatches.

A separate detailed storm warning issued at 2.35pm said storms on the radar in the Coalfields were headed east.

"They are forecast to affect Newcastle City, Raymond Terrace, Wallsend and Medowie by 3:05 pm and Nelson Bay, Anna Bay, Lemon Tree Passage and Karuah by 3:35 pm," the warning said.

A 43km/h gust of wind was observed at Narrabri about 1.30pm.

It follows huge weekend swell that battered the Newcastle breakwall.

Meanwhile, traffic was heavy for a number of hours on Nelson Bay Road near Fullerton Cove Monday afternoon, after a caravan was blown onto its side in severe winds.

NSW Transport Management Centre advised crews were working to clear the roadway, but drivers were told to expect delays. Southbound lanes re-opened around 4.45pm. It is understood no one was injured as a result of the crash.

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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.

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