Crews were being assisted by two helicopters helping to dampen hotspots.
The warehouse, stocking various forms of books, stationery and office furniture, is surrounded by similar "big box" stores, said Mr Osmond.
Firefighters this morning were trying to stop the flames setting alight the neighbouring buildings, and were based on the Amohau St side of the industrial area.
Mr Osmond said the fire reached fourth alarm, which was a high ranking but not the worst.
He said there had been no reports of injuries that he was aware of, and there had been no one inside the building.
It was not yet known how the fire started.
Police are at the scene, and are asking all pedestrians and traffic to keep well clear of the blaze.
The police have asked people to avoid the area bounded by Amohau, Fenton, Victoria, and Ranolf streets.
There had been no reports of injuries.