The fire broke out at about 4.20pm yesterday and completely destroyed the top level of the Geelong Street building.

Smashed windows at the premises revealed the extent of the damage, the interior of each room was completely black and the heat had cracked the brick exterior.

A cafe on the ground floor was not burnt, but sustained significant smoke and water damage.

Three pumpers, a breathing apparatus vehicle and the Bronto aerial appliance were called to the scene.

It has not been discovered where the fire started, nor has a cause been determined.

Fire investigators were called in last night but held off beginning their inquiries until this morning.

The fire is believed to be suspicious at this stage.

The Canberra Times understands the building had been recently sold.

Speaking at the scene, Acting Superintendent of Fire Operations Ron Weston said one woman, the property's owner, was inside at the time.

She was interviewed by fire investigators at the scene.

Superintendent Weston said it was unclear at this stage whether the building would have to be demolished.

"The damage to the top level is significant but we will get structural experts to determine whether the integrity of the building has damaged."

The ACT fire brigade was maintaining a constant presence at the scene overnight in order to keep the premises secure.