UPDATED 7.08pm: A large fire is burning in Christchurch CBD, on the corner of Tuam and Colombo Streets.

The blaze has now been categorised as a third alarm.

A spokeswoman for the Southern Fire Communications said they were dealing with "an event'' and could not take any calls at the moment as a result.

Onlookers say it looks like an abandoned warehouse is engulfed.

Fire crews are attending, and Lichfield and Tuam streets have been closed between Colombo and Manchester streets.

A spokeswoman for St John confirmed they received a call to assist at about quarter past five.

Motorists are being warned to expect significant delays in the area.

Authorities are telling motorists to avoid the central city.

"Motorists are recommended to use Moorhouse Ave as an alternative to Tuam St and Lichfield St."

Witness Abby Gilligan said at first she thought the smoke was coming from the bus exchange, before seeing the warehouse next to where Real Groovy record store used to be.

"And that's when I saw more smoke billowing out and just when I passed it flames erupting from all the windows."

Ms Gilligan said she could smell the smoke from her car and the fire seemed to have started deep inside the building.

Large plumes of black smoke can be seen across the city.

Video footage being uploaded to social media by members of the public shows big flames coming from what appears to be a large warehouse or factory building.

The area is of the old Sol Square in central Christchurch.

Firefighters can be seen around the building, as several fire crews and police cars line the street immediately outside.