Four appliances were called to the scene about 2pm, the blaze taking about an hour to extinguish.
Publican Piki Moore of the Grand Tavern across the road said she saw flames shooting out from the roof of the building.
"There was quite a bit of flame and smoke coming out.
"And the young lady who runs LJ Hooker came running out quite smartly. I’m not sure if she rang the fire brigade or Fulton Hogan did because we had a truck here.
"A lot of people were just looking at awe at all the smoke and flames."
A Fire and Emergency New Zealand spokesman said the blaze was fully involved and coming from the roof from a commercial building on Kelso St.
A police spokeswoman said a unit initially assisted with traffic management.
The cause of the fire was unknown and a fire investigator had been assigned to do an assessment, the spokesman said.
