A Christchurch factory is back in business just two days after a spectacular fire.

The blaze damaged the roofs of four storage buildings at the Ravensdown fertiliser factory on 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.

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Around 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," 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.

The fire damaged four out of Ravensdown's 13 stores on its site. Photo / Supplied
The fire damaged four out of Ravensdown's 13 stores on its site. Photo / Supplied

"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 exact 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 doing the maintenance work.

"We want to acknowledge the concern of the wider Christchurch community and disruption caused to our neighbours. We'd like to thank them for their patience and the many well-wishers for their support," Campbell said.

After a massive effort from the team to make the site safe to access, customers will be able to order and collect product from 8am this morning.

Campbell said Ravensdown had good stocks of finished fertiliser products for the coming season. He said in the initial restart of service, customers could expect some congestion on site.

"As a proud member of the Hornby community since 1922, we are determined to rebuild, restore and improve the site as it eventually becomes fully operational."