NewstalkZB reporter Jess McCarthy said the blaze appeared to be under control by about 6am.
"One of the fire trucks does have its crane arm up, so potentially it was shooting water down on to the roof," she said.
"It is smoking upwards. There are not actually any signs of flames."
Police were called in to help evacuate a nearby motel as a precaution, a spokeswoman said.
Officers are also in the area helping to control traffic.
Motorists in the area are being told that the section of Fitzgerald Ave northbound, from Lichfield St, has been closed off as a result.
Cashel St is also closed to traffic mid-block from Barbadoes St.
Transport for Christchurch issued an alert via its social media channels saying Fitzgerald Ave is, however, open to traffic heading south.
"Please avoid these routes until clear," the alert said.