A large factory fire that extensively damaged a factory in Christchurch is out.

The roof and side of the building were badly damaged, and fire appliances and a fire investigator are still at the scene.

Main South Road, from Carmen Road to Springs Road is now open.

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

Forty firefighters and 10 trucks battled the blaze at the Ravensdown fertiliser factory on Main South Road, Fire and Emergency NZ says.

The blaze began in Main South Rd.

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 CEO Greg Campbell said there had been no injuries and everyone was accounted for.

"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."

They didn't know the cause of the fire, and would be investigating.

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

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