Walgett and Collarenebri have been waiting for water from ex-tropical cyclone Oswald to get to them after the NSW State Emergency Service (SES) Macquarie Region sounded the alarm early last week.

The river may peak at 6.8 metres today at Collarenebri, and at 10.7 metres at Walgett on Tuesday, causing minor flooding in both locations.

At Collarenebri, 75 kilometres north-east of Walgett, part of Maitland Street and the Gwydir Highway east of the town are set to close.

"As a result of a further upwards revision of the predicted peak height by the Bureau of Meteorology, it is likely floodwater will cross the Gwydir Highway , SH12, between Collarenebri and Moree, about five kilometres east of Collarenebri," Dubbo-based incident controller David Monk said.

"It is expected that the road will close at the predicted height.

"People using this road should continue to monitor the situation."

The SES predicts water would fill low-lying areas of the road to Walli Reserve, but that it would remain open and drivers of vehicles with high-clearance should be cautious.

Walgett is being told to prepare for minor flooding outside of its levee, closing part of Wee Waa Street and parts of Brewon Road.

Downstream of Walgett, river levels along the Barwon and Darling system are not expected to reach minor flood level.

Major rural flooding was reported yesterday along the Barwon River at Mogil Mogil, where the peak arrived on Wednesday.

The SES told people working or camping along the Barwon River downstream of Mungindi on the NSW and Queensland border, that they "should have already moved to higher ground".

It continues to urge farmers and landholders to move livestock, pumps, machinery, waste and chemical containers.

Farmers can seek help from their Local Health and Pest Authority ranger, or Industry and Investment NSW. People expecting to be isolated by the flooding are being encouraged to stock up on essentials.