The Fire Service in Napier says it took more than 40 firefighters about an hour to extinguish a blaze in the Ravensdown fertiliser storage building on the northern outskirts of the city.
There are no reports of injury.
Incident controller Chris Nicol says the fire broke out about 11.30am on Tuesday in the industrial area of Pandora.
"The fire appeared to start in the conveyor belt which runs along the ridge of the buildings lengthwise," he says.
"We were unable to access that, simply because the roof was too steep and it was too dangerous to put people inside the building at the time. So we had an external attack, dampening down where we could from the high areas of the roof where the fire was breaking through."
Nine engines were sent to the fire.
Fire safety officers are investigating the cause.