Two cleaners raised the alarm about a fire after a light apparently exploded at Tait Communications' Christchurch office.
A fire, which engulfed the storage building, has been brought under control.
Dozens of firefighters had been fighting the blaze, but were now dampening down hotspots.
Fire Service spokesperson Andrew Norris said the blaze reached "fourth alarm" status, meaning eight engines, a command unit and a turntable ladder were there, he said.
"We've got a fire at a building of approximately 300sqm, it's a two-storey building used for storage," he said.
He said, with three other buildings nearby, firefighters worked defensively to contain the blaze to a single building.
The building, which was not part of the company's manufacturing operations, had been badly damaged.
Two staff had been unaccounted for when the blaze started about 6pm, but had since been found safe.
Tait Communications chief executive Garry Diack said he was relieved no one had been hurt.
He said it had not been confirmed how the fire started, but it began in the building's office suite.
"It's been reported by the two cleaners that were in the building that a light had exploded in the ceiling but htat's yet to be verified by the authorities.
"They were in there, they heard the explosion, and then they activated the emergency response process which resulted in the fire department arriving pretty quickly."
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