Chef lit pilot light but can't remember any more.

Chef lit pilot light but can't remember any more. Photo: Melissa Adams

Disruption to Canberra's central business district will continue for at least another day as investigations into Monday's fire in the historic Sydney Building get under way.

Fire crews extinguished the fire about 2am on Tuesday, and ACT Policing took control of the site from ACT Fire and Rescue about 3pm.

Police, the coroner and fire investigators are looking into the cause of the blaze.

Road closures on East Row and disruptions to ACTION Bus services from the Civic Bus Interchange are set to remain in place as structural engineers and forensic investigators inspect the burnt-out premises.

As many as 40 businesses on East Row and inside the historic building are affected by the investigation, with many set to be closed for a third day on Wednesday.

WorkSafe ACT air monitoring continued on Monday night, ruling out the immediate risk to the public from asbestos fibres.

ACT Policing officers remain at the scene to provide security for businesses in the fire's immediate vicinity. Onlookers watched the operations on Tuesday, taking photos of the burnt-out Coo restaurant.

''We're now commencing the investigation phase, but there are significant structural challenges so we have to wait until those are dealt with before we can investigate,'' ACT Fire and Rescue commander Michael Hobbs said.

''It was a restaurant; there is a kitchen area in there, so it's possible the kitchen was involved, but there's no way to tell at this point. The damage to the unit is too extensive.''

He said the involvement of the coroner was a standard aspect of a large fire investigation.

''The only business we can't enter at this stage is the fire-damaged unit. All the other units have suffered either smoke or water damage but they are structurally intact,'' Commander Hobbs said.

ACTION Buses chief operating officer Bren Burkevics said alternative bus stops on Northbourne Avenue, Alinga Street and Marcus Clarke Street would remain in place until East Row was reopened in coming days.

''Bus stops along London Circuit East, including at the Legislative Assembly, the Waldorf and the Commonwealth Bank, were reopened for all services at 2pm on Tuesday,'' he said.

''As soon as we get advice from WorkSafe ACT, we will advise the public of when the bus station will return to full or partial operations.''

WorkSafe ACT Commissioner Mark McCabe said staff were contacting other business owners locked out of their premises.

''The main issue is the structural integrity of the building. The other very important issue is the loose tiles that are sitting on top of the roof as a result of the fire.

''With some winds being predicted for tomorrow, we need to secure that roof before we can go further.''

Commissioner McCabe asked all affected business operators from the Sydney Building to contact WorkSafe ACT to discuss arrangements for getting back into the building.