A CBD apartment building hit by fire on New Year's Eve is one of the many Melbourne buildings known to harbour illegal rooming houses, with living rooms rented to multiple tenants as sleeping spaces.

But a City of Melbourne building surveyor who inspected the apartment where the fire broke out said there was no evidence that particular flat was overcrowded.

More than 27 firefighters from 12 fire trucks battled the blaze on the 29th floor of 200 Spencer Street, opposite Southern Cross Station.

MFB crews managed to contain the fire to an air conditioner on a balcony before bringing it under control. Firefighters received a report of the blaze at 1.48pm.