Plumes of black smoke were seen across the city and explosions heard by those nearby as a fire took hold in the Ravensdown factory on Main South Rd, Hornby.

A Christchurch fertiliser factory ravaged by fire is expected to cost "many millions" of dollars to repair, a company spokesman says.

Explosions were heard and thick plumes of black smoke were seen across Christchurch as the fire took hold of the Ravensdown factory on Main South Rd, Hornby, about 10.15am on Tuesday.

Firefighters stopped the blaze just metres from a sulphur store, preventing further evacuations in the surrounding area.

STACY SQUIRES/STUFF Evacuated staff from nearby Iplex Piplelines NZ watch the fire from Braeburn Drive.

A Ravensdown spokesman said on Tuesday evening the exact cost of the damage was not yet known but it was expected to be "a multimillion-dollar fit out".

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"I wouldn't like to determine how many millions but we will have a better idea [on Wednesday]."

DAVID WALKER/STUFF Flames and smoke pour from the roof of the Ravensdown factory on Main South Rd on Tuesday.

Engineers and electricians would be on-site Wednesday morning to determine the full extent of the damage and the associated cost, the spokesman said.

"We want to make sure there is zero risk before making a judgement on reopening."

An initial assessment indicated it was "looking good from a business point of view", the spokesman said.

SUPPLIED A large fire has erupted at Ravensdown's fertiliser factory in Hornby, Christchurch.

None of the fertiliser products had been damaged and the company still planned to provide its spring and stored products, he said.

The fire did not damage the office building and it would operate as normal on Wednesday, he said.

Ravensdown staff members would also be talking to concerned residents on Wednesday.

DAVID WALKER/STUFF A firefighter douses flames and smoke pouring out the roof of the factory.

The spokesman said he would like residents to know no asbestos buildings or fertiliser was involved in the fire.

"We really acknowledge the concerns of the residents, we will be investigating to stop this happening again."

A Fire and Emergency New Zealand spokesman (Fenz) said about 40 firefighters and 10 trucks worked to contain the blaze, which burned through framing and cladding.

GEORGE HEARD/STUFF Fenz assistant area commander Steve Kennedy Steve Kennedy says firefighters did a "great job" in containing the fire so quickly.

Maintenance work was being carried out on the building when the fire happened. The smoke seen across the city was mostly created by burning rubber from a conveyor belt. No chemicals were involved, the spokesman said.

Fenz assistant area commander Steve Kennedy said the conveyor belt ran through the length of the building and helped spread the fire through about a third of the roof structure.

"Once we got a fire up in the framing of the building and the roofing and the exterior cladding, it's going to catch on to that conveyor and spread along through there."

STUFF The cause of a factory fire that could be seen from across Christchurch on Tuesday is yet to be determined.

Firefighters did a "great job" in stopping the fire spreading into a nearby sulphur store.

"It came within metres of the sulphur store, which would have required [further] evacuations."

The fire was contained not long after 1.30pm.

GOOGLE MAPS Part of Main South Rd has been closed while emergency services deal with a fire at the Ravensdown factory.

Fire investigator Mike Gaskin said an investigation would continue over the next few days until investigators gained access to certain areas of the factory.

"It's looking like it's an accident at this stage but we haven't been able to eliminate every hypothesis to say definitively," he said.

Ravensdown chief executive officer Greg Campbell said it was unclear what caused the fire, which started at the end of the storage store, but staff on site told him it could be related to the conveyor belt. The 14-hectare plant stored and manufactured various agricultural products. It did not house any nitrates or explosives, he said.

GEORGE HEARD/STUFF Ravensdown chief executive Greg Campbell says all staff at the factory are safe.

Fifty-seven staff were evacuated from the facility as a "precaution" and staff were sent home for the day.

The company was "fortunate" to have two other plants, one in Dunedin and another in Napier.

"We acknowledge any inconvenience to our neighbours and we'd like to thank the well-wishers for their support."

STACY SQUIRES/STUFF Workers from a nearby company watch the fire from Braeburn Drive.

Ravensdown was already working with its customers and transporters to help them with alternative supply options, Campbell said.

Ravensdown has operated at the site since 1922.

'BIG BOOM' HEARD

GEORGE HEARD/STUFF Firefighters work to prevent flare-ups on a burnt-out section of the Ravensdown factory.

Hairstylist Hannah Kearney, who works nearby, said she spotted a "huge pile of black smoke" across the road just after 10am.

"I ran over and let [Ravensdown] know. They didn't realise how big the fire was.

"I walked down the road and I saw flames about 2 metres above the building. Then I heard an explosion – like a big boom," she said.

GEORGE HEARD/STUFF Firefighters at the scene of a fire at Ravensdown in Christchurch.

A nearby worker, who did not want to be named, said "really black smoke" was surrounding the building.

She heard several small bangs shortly after firefighters arrived at the scene.

Canterbury medical officer of health Dr Cheryl Brunton said anyone who experienced respiratory issues from the smoke should contact their GP first, but phone 111 for an emergency.

SUPPLIED/PHIL MEIKLE Flames can be seen burning through a building at the Ravensdown factory on Main South Rd, Hornby.

CHRISTOPH BARTNECK Plumes of black smoke billow into the sky around the factory.