The fire started in the plant's slaughter and boning rooms.
The Fire Service in Waikato says a contractor installing a vent on Friday afternoon cut through the external cladding and internal panels to fit the vent.
It says when the work was sealed up, a spark from the blade must have started to smoulder in one of the panels.
A Fire Safety Officer, Kevin Holmes, says it would have begun as a minor fire that smouldered for hours before becoming an inferno, and he says a sprinkler system would have saved the building.
