The State Emergency Service on Friday morning told residents of Boggabilla and Toomelah they must evacuate by 11am or risk being stranded without help.
"Once floodwater enters the area road access will be cut and all sewerage and power will be lost," the evacuation order warns.
"If you remain in the area after 11am, you will be trapped and it may be too dangerous for SES to rescue you."
An evacuation centre has been set up at Moree East Public School.
On Friday afternoon the Newell Highway between Boggabilla and Moree was closed by flooding.
Severe storms have been threatening areas around the Greater Sydney region, with a band developing from the Hunter through the Blue Mountains and down to the Illawarra region on Friday afternoon.
The Bureau of Meteorology warns Gosford, Cessnock, Maitland, Bowral, Katoomba and Taralga are among the areas that could be impacted by the damaging winds and large hailstones on Friday afternoon.
A "very dangerous" storm in Penrith was due to move towards Richmond, while another at Picton was moving southeast towards Wollongong.
Another storm was moving northeast over the Hunter region.
The Macintrye River has already reached moderate flood levels in Boggabilla, but is expected to peak with major flooding.
Major flooding is also occurring upstream in Goondiwindi, with the river likely to exceed the peak of the 2011 floods that devastated southeast Queensland.
But the border town is "incredibly prepared" for floods, Queensland Fire and Emergency Services Assistant Commissioner Steve Smith says.
"There's obviously a levee in place," Mr Smith told the Today show.
"The expected peak is 10.7m. The levee is sitting at 11m."
The Macquarie River is expected to reach a 9.5 metre peak before beginning to recede overnight at Warren, where major flooding has isolated the town.
Major flooding is also underway at Wee Waa along the Namoi River and at Jemalong on the Lachlan River.
Flood warnings are still active for the Severn, Gwydir, Dumaresq, Bogan, Murrumbidgee, Murray, Warrego, Paroo and Barwon rivers, after NSW's wettest November on record soaked catchments.
But sunny weather has returned for a spell, with multiple parts of NSW facing maximum temperatures exceeding 30C on Friday.
Federal-state disaster assistance is available in 39 local government areas across NSW for individuals and councils to clean up damage from November's floods.
29°C