Temperature records tumbled across NSW amid heatwave conditions two weeks ago, but this weekend will be much cooler and wetter for the state's mid-north coast and northern rivers regions.

A severe thunderstorm warning was issued on Friday evening for Moree, Narrabri, Walgett, Bourke, Lightning Ridge and Brewarrina in the state's northern interior.

The Bureau of Meteorology's warning said heavy rainfall is expected that may lead to flash flooding, as well as large hailstones and damaging winds.

The warning was cancelled around 7pm, after the worst of the storms passed.

It comes as areas of the mid-north coast brace for heavy rainfall over the next 24 hours, on top of almost the 100 millimetres of rain which fell overnight on Thursday.

The coastal trough causing the havoc will then enter southeast Queensland late on Saturday, the Bureau says, before strengthening and returning to NSW on Monday.

When it returns, it will bring strong winds and dangerous sea conditions.

"We're at quite a high part of the tidal cycle so (with) those waves in combination with the high tides, we're looking at potentially some inundation of some low-lying areas and potentially some coastal erosion up in the north," the bureau's Jane Golding told reporters.

Flooding of rivers in the region is likely from Saturday.

"At the moment we're closely watching the catchments between the Tweed and the Hastings, so that's from the Queensland border down to around about Port Macquarie," Ms Golding said.

The NSW State Emergency Service has briefed and allocated extra volunteers to the region to deal with the weather.

"We've deployed some extra volunteers to those locations including flood rescue technicians," SES Chief Superintendent Greg Swindells said.

"We're expecting we will see a lot of people in orange uniforms out and about helping the communities in which they live."

Australian Associated Press