Huge fire at Ravensdown fertiliser factory in Hornby, Christchurch

Explosions can be heard and huge plumes of thick black smoke seen across the city from a massive fertiliser factory inferno in Christchurch.

Forty firefighters and 10 trucks have been battling the blaze at the Ravensdown fertiliser factory on Main South Rd, Hornby, since 10.15am.

The blaze began in Main South Rd.

Consultant Amy Redman said she was next door at the Iplex building when they saw small flames coming out of the factory roof.

"Minutes later the whole roof was set alight. I have never seen anything like it, I couldn't believe it was real.

"The fire then looked like it went inside the building and we started hearing some small explosions. Everyone moved back and out onto the road.

"Everyone was in shock as they were evacuating the surrounding sites."

Redman said the thick smoke could be smelled everywhere.

"It smelled really intense."

Lucas Perelini said it looked like the whole building had gone up in flames.

"I was standing about 400 metres away, but could hear it hissing and loud bangs like small explosions. It's pretty hectic."

The wind was blowing the thick black smoke away from where he was standing, but it "didn't look good".

Ravensdown said all staff were accounted for and no injuries had been reported.

"Ravensdown confirms there is a fire at its intake on the Hornby premises. Emergency services are in attendance and we are currently working with them."

CEO Greg Campbell said they didn't know the cause of the fire, and would be investigating.

A Christchurch Transport Operations Centre spokeswoman said motorists should expect severe delays.

"If you are travelling east or west to or from SH1 please use the Christchurch Southern Motorway as an alternate route.

"If you are travelling from Sockburn Roundabout use Racecourse Rd and Buchanans Rd."

Black smoke can be seen across the city. Photo / Rachel Das

Main South Road is now closed from Carmen Rd to Springs Rd.

"The public are urged to avoid the area," police said.

Black smoke can be seen across the city. Image / NZTA