The out-of-control fire has grown to 442-hectares in the Nundle State Forest, with its warning level upgraded to watch and act.

Tamworth RFS operations officer Steven Prior said a number of factors, including the terrain and weather conditions, had made it tough for the dozen crews fighting the fire.

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"It is extremely challenging due to the very heavy fuel loads in the forest, along with the extreme lack of moisture in fuel and soil below it," he said.

"Those factors has made the fire intensity very high, which creates the problem of embers becoming airborne and spreading ahead of the main fire.

"It makes it very difficult to keep the fire within the containment lines and it has already jumped them a number of times today."

Mr Prior said three aircraft were airdropping the fire, while four bulldozers and two excavators were flat out creating containment lines.

"It is a very, very difficult situation, simply because of the fire intensity and the prevailing conditions," he said.

"Because it's elevated, the wind goes up the hills, contributing to the spread of the fire.

"It's basically fanning it ahead, but not constantly in the same direction or speed. It depends on the wind strength which is variable and changing."

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Mr Prior said what firies really need is "for the weather to turn around".

"That doesn't look likely in the foreseeable future," he said.

Crews also fought a small but challenging fire near Mulla Creek. The seven-hectare fire was started by a lightning strike.

Because of the terrain, crews couldn't get a truck to the blaze and had to use hand tools to dig a containment line.

Neighbouring properties used earth moving equipment to create a break on the northern and western sides, while backpackers from the nearby jillaroo school also helped out.

The Den Mountain fire near Watsons Creek is still burning after two weeks, with the "extremely inhospitable terrain" again making it difficult for crews to "get to the point to call it contained".

"We're working day and night on that," Mr Prior said.

"We've had one or two dozers working constantly over the last week with water bombings, to try and keep the fire in place until they finish their work.

"We're in the later stages of backburning as conditions allow."