The Duck Creek-Pilliga fire south of the town of Narrabri has already ripped through more than 80,000 hectares.
The state’s Rural Fire Service overnight upgraded its advice to residents, with warnings lives are at risk in parts of Bohena Creek.
“A fire-generated thunderstorm has formed causing dangerous fire behaviour,” the RFS said.
“If you are in Baan Baa, Willala or Goolhi your life is at risk. Seek shelter shelter now in a solid structure such as a house.
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“Do not be caught in the open in the path of the fire.”
The fire within the Pilliga Forest, earlier reached an emergency level and has burned through more than 80,000 hectares.
About 68 firefighters and multiple aircraft have been working to contain the blaze, which was burning in a north-easterly direction towards the Newell Highway.
Narrabri is about 520 kilometres north-west of Sydney.