ACT Fire & Rescue was called to the scene at 3:55am and arrived to find significant damage to the restaurant, which a business owner across the road said had been empty "for some time".

The site on the corner of Franklin and Furneaux streets was most recently home to Avesta, a Mediterranean restaurant, and previously housed Pheast and Charlie Black's Brasserie.

An ACT Emergency Services Agency spokeswoman said firefighters were checking damage to surrounding businesses after extinguishing the fire in the premises.

The restaurant was completely gutted and police also reported smoke damaging to neighbouring properties.

An ACT Policing statement said that a man was seen riding a bicycle in the area about the time of the fire, and urged that man and any other witnesses to come forward with information.

A crime scene will remain in place throughout today and AFP Forensic Services will conduct an examination of the scene.

Four pumpers, the breathing apparatus van and two commanders attended the scene, and crews were still undertaking ventilation operations early Wednesday morning.

The ACT ESA spokeswoman said the cause was not known but firefighters had handed the investigation to ACT Policing, which was treating it as suspicious.

Police urged any witnesses to call Crimestoppers on 1800 333 000.