A Fire and Emergency New Zealand (Fenz) spokesman said five appliances and an aerial unit, 22 firefighters in total, were called to a fire on the top floor of a building in the Octagon, at about 3.30pm yesterday.
There were no reports of injuries.
Scott Evangelou, of Buy Bullion, whose offices are on the fifth floor of the Moran Building, said there had been an electrical fire in one of the overhead lights in the lobby.
The fire in the light was "melting away" the plastic cover, and a person on the fourth floor noticed flames before people began evacuating.
"It was just dripping down all the way down the stairway, like hot plastic," Mr Evangelou said.
Gelato Junkie worker Maia Puricelli said they were working hard when people began to notice a fire on the top floor.
At the scene, Senior Station Officer Michael Johnston said crews extinguished a small fire on the top floor of the building, using a hose reel.
The building was fully evacuated and a fire investigator was on scene to determine the cause.
Crews had done a good job of keeping the fire contained and extinguishing it quickly, SSO Johnston said.