A fire service investigation has determined the huge factory fire in the Auckland suburb of Onehunga was accidental.
Five people were left with minor injuries after yesterday's blaze and the textile factory building was severely damaged.
People nearby said they could see flames reaching "one of two storeys high" and black smoke billowing from the factory.
Fire safety officer Ray Coleman said a rogue piece of metal that got caught in machinery, causing sparks, triggered the fire.
He said workers discovered the fire in a storage bin which catches fibre from a shredder.
"They opened the door and saw the fire in the storage bin, they got a hose and tried to extinguish the fire," Mr Coleman told Fairfax.
"Unfortunately the fire was well established and spread to other material nearby and ultimately they just had to abandon what they were trying to do and get out of there until we got there."
He said workers should have called emergency services as soon as the fire was discovered.
Sprinklers were not installed in the factory.
Mr Coleman said sprinklers could have contained the fire and would have instantly alerted the fire service.
NZN