The BestCare Foods Limited plant in Borthistle St in the town's industrial area on the western edge of the town, was reduced to little more than a smouldering skeleton of twisted metal after the enormous boiler explosion and subsequent fires.

Damage to the factory, business and houses in a 1km radius is expected to run into millions of dollars.

The blast, just after 7.50pm, was felt up to 20km away, with homes in the area being shifted off their foundations and many having windows blown out.

As well as the plant and homes, the Namoi Division SES headquarters was severely damaged.

Emergency services cannot believe that the only injury reported was from a man who cut his hand while checking on a friend's well-being.

The cause of the blast is believed to have been a boiler which, when it exploded, set off other explosions in LPG cylinders.

Dust from wheat used in the production process also contributed to the intensity of the explosion and fires.

Residents reported seeing a mushroom cloud over the factory site and small explosions and debris flying through the air shortly after the firstexplosion, which lifted houses and business from their foundations, shattered windows, bent industrial roller doors and ripped airconditioning units from roofs.

A full-scale emergency procedure was adopted and residents from surrounding homes were evacuated immediately.

Emergency Services received a number of calls shortly after the initial explosion and 35 firefighters from crews around the Gunnedah, Tamworth and Narrabri divisions of the NSW Fire Brigade joined the Rural Fire Service, SES and an ambulance on standby shortly after the explosion.

NSW Fire Brigade operational commander Inspector Steven Pearce said it was the largest explosion he'd seen in the area and it was fortunate that no one was killed or injured.

"At this stage investigations are still continuing to determine the cause of the explosion, but initial reports suggest that it was a build up of pressure in one of the boilers," Inspector Pearce said.

"Following the first explosion, which caused an enormous amount of destruction, a number of LPG gas cylinders continued to explode.

"The hazardous materials unit was brought in after fears of harmful toxic chemicals might have been a problem, but later investigations revealed the smoke from the fire was a minor concern."

Residents in a 500 metre radius of the explosion were evacuated from the area and were told not to return until the morning.

"Although the smoke from the explosion wasn't a major concern, we still asked residents to leave the area to be on the safe side," Mr Pearce said.

Four fire engines tackled a number of small explosions and fires following the initial explosion with measures put in place to ensure water run off was contained and monitored.

"Because of the possibility that toxic chemicals could have been inside the factory after the height of the explosion the EPA closely monitored the run off of water and storm water drains surrounding the area," Inspector Pearce said.