Fire investigators are using a drone to help piece together the cause of a large fire that destroyed a Dargaville plastics factory.
The blaze at Robertson Plastic Factory on Tuesday night was not suspicious but possibly the result of an electrical issue, fire investigator Craig Bain said.
"We've eliminated what it's not. It's definitely not suspicious. It's an accident," he said.
Nine fire appliances and more than 60 firefighters responded to the blaze which broke out at about 9.30pm.
While the definite cause of the fire would be identified next week, initial investigations indicated it was "possibly an electrical issue", Mr Bain said.