Fire in Sydney Building restaurant in Civic

ABC Firefighters have forced entry to a restaurant in the bus interchange.

Police and work safety inspectors will investigate a large blaze in the historic Sydney Building in Canberra's CBD.

Investigators say there is nothing to suggest the fire is suspicious but they do have concerns that a gas leak might have caused an initial explosion.

The building, which is one of the oldest in Canberra, has been significantly damaged by the large fire.

It is believed the fire started in a Japanese restaurant in the Sydney Building at about 9:45am.

Thick, dark smoke has billowed high above the city skyline as the fire moved from the ground floor restaurant into the roof.

Neighbouring business owners reported hearing a large explosion and seeing the restaurant's shop frontage blasted out.

"We just heard a loud bang and we came out to see what had happened," a chef at a nearby cafe, Peter Spencer said.

"The front of the shop had popped out from the explosion."

Office workers from around the CBD watched the blaze from behind police safety lines, with many posting pictures of the fire and smoke on social media.

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Fire in Sydney Building restaurant in Civic