The Duck Creek fire in the Pilliga Forest was burning about 17km south of Narrabri and 21km west of Boggabri early on Tuesday, having burnt more than 84,000 hectares.
The Rural Fire Service warned in its latest alert that embers would be blown a long way ahead of the main fire front and could set homes alight.
It said firefighters were working to protect properties and slow the spread of the fire.
"Overnight the fire generated a thunderstorm (pyro-cumulonimbus) to the southeast of the fireground, causing dangerous and erratic fire behaviour," the RFS alert said.
Residents of Baan Baa, Willala and Goolhi have been warned their lives were at risk and it was too late to leave so they should seek shelter and not be caught in the open in the path of the fire.
People in Bohena Creek, Jacks Creek, Rocky Glen and Wynella have been warned to stay alert and monitor conditions.
Evacuation centres have been set up at Narrabri and Gunnedah.
Smoke from the fire on Monday could be seen across an area spanning hundreds of kilometres including Tamworth, Walcha, Armidale and even coastal communities near Port Macquarie.
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