Severe thunderstorms with large hail, heavy rainfall and damaging winds are forecast for parts of inland NSW and southern Queensland on Wednesday, Bureau of Meteorology Senior Meteorologist Jenny Sturrock said.

A severe weather warning has been issued for the state's central west slopes and plains, with six-hourly rainfall totals of up to 60 to 70mm expected, triggering a risk of flash and riverine flooding.

Flooding is already impacting Dubbo, Gunnedah and Tamworth, with Parkes, Wellington, Coonamble, Gilgandra and Peak Hill also at risk.

Helicopters are preparing to fly in supplies to the town of Wee Waa, between Narrabri and Walgett, as flooding cuts off road access.

A strong western front is expected to fill the already swollen Namoi River on Wednesday evening, stranding the town with a population of about 2000.

The Lachlan, Narran and Macquarie Rivers are already experiencing major flooding.

Further rainfall across Wednesday night was likely to exacerbate flood conditions throughout NSW and down to northern Victoria and up to southern Queensland, Ms Sturrock said.

Falls are likely to concentrate around the coast on Thursday.

"Heavy rainfall for the Hunter, the Mid North Coast and even the Northern Rivers can't be ruled out."

Light to moderate falls across saturated catchments have the potential to trigger significant flood responses, and Ms Sturrock urged the community to stay up to date with warnings from the BOM.

Many catchments in the northwest, central west and southwest are already dealing with flooding after heavy recent falls saturated catchments and filled dams.

Residents in northwestern Gunnedah and Wee Waa are facing their third flood in a year as five houses were inundated when the Namoi River peaked at over eight metres on the weekend, the State Emergency Service said.

In the the state's central west, Dubbo received 30mm of rain by 3pm, on the lower end of the predicted rainfall.

"The intensity of rain we've anticipated hasn't fallen yet," SES spokesperson David Rankin told AAP.

"That's really good news but we aren't out of the woods - it could come later tonight."

The SES urged people living in flood warning zones to prepare for the potential of potential flash flooding.

Residents from the Coffs Coast, Kempsey and Nambucca, as well as those further south at Coutts Crossing and the Upper Hunter, should prepare for rising waters and monitor for road closures.

"NSW river catchments remain wet at present and any storm or weather event can lead to flash flooding very quickly," SES Superintendent Joanna Jones said.

She urged people not to enter flood waters and to update their emergency plans.

People in inland NSW should prepare to leave homes that could be inundated and move pets and livestock to safe higher ground, she added.

Australian Associated Press