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The blaze damaged the roofs of four storage buildings at the Ravensdown fertiliser factory in Main South Rd, Hornby, on Tuesday morning.
The fire, which damaged four out of Ravensdown's 13 stores on its site, appears to have started during some maintenance work and spread along the roof line when a rubber conveyor belt ignited.
Explosions could be heard and huge plumes of thick black smoke were seen across the city.
About 40 firefighters fought the blaze and stopped it just metres from a sulphur store.
Chief executive Greg Campbell said the rubber belt helped spread the fire and caused the black smoke.
"The buildings affected were of new fibreglass construction and did not contain any asbestos.
"The cladding responded as it should, allowing emergency services to put the fire out quickly and safely from outside the building," Mr Campbell said.
"The response of our staff was exceptional, ensuring the site was safe for the emergency response team.
"I also want to thank the emergency services for the way in which they contained the fire so quickly.
"Nobody was injured and buildings are insured and will be replaced.
"The contingency planning and emergency response of the team has been outstanding as demonstrated by being back up and running so quickly."
He said the fire only affected building materials and conveyor structures.
The cost of the damage was not yet known but it is expected to be a multimillion-dollar fit out.
Ravensdown does not store explosive materials in any of its manufacturing plants.
Two small bangs were likely caused by gas bottles being used for the maintenance work.
After a massive effort from the team to make the site safe to access, customers were able to order and collect product from 8am yesterday, Mr Campbell said.
Ravensdown had good stocks of finished fertiliser products for the coming season.