Bus wasn't fitted with new fire safety before Sydney Harbour Bridge blaze

The bus that caught fire on the Sydney Harbour Bridge did not have an automatic fire suppression system fitted, transport authorities have revealed.

The State Transit Authority of NSW confirmed the bus is one of just 300 vehicles not yet fitted with a fire suppressant system, forcing passengers to flee the burning bus and onto the busy roadway.

The driver of the bus has been released from Royal North Shore Hospital, hours after receiving treatment for minor injuries he sustained in ensuring all passengers made it safely off the bus.

Two of those passengers were also taken to Royal North Shore suffering smoke inhalation while 11 others made it off the bus unharmed.

In August the State Government announced all 2200 buses in its fleet would be retrofitted with the new fire safety after a series of fires.

So far, 1900 buses have been upgraded but the 20-year-old bus that caught fire on the Harbour Bridge was not one of them.

The fire caused traffic chaos in Sydney with all northbound lanes shut down after the Milsons Point blaze during peak hour on Thursday afternoon.

Flames and thick smoke billowed from the bridge as firefighters worked to control the fire, which was extinguished around 5.30pm.

A tow truck removed the scorched bus from the bridge an hour later, with traffic banked back around 8km as several lanes began to reopen.

Motorists were urged to find alternative routes with extensive delays on all approaches to the busy arterial road.