A RAGING bus fire that halted traffic on Sydney Harbour Bridge for hours on Thursday night is being blamed on the state's transport minister and his failure to implement fire-control systems on the vehicles.

The Rail Tram and Bus Union is claiming the fire -- which began at 5.30pm and delayed commuters during the evening peak hour -- should not have happened.

Harbour Bridge blocked by burning bus.
Harbour Bridge blocked by burning bus.

Union secretary Chris Preston said the government and the minister had been too slow to ensure "fire suppression systems" were in place on all buses.

"Buses do catch alight. If they have fire suppression systems fitted to them, they will not catch alight," Mr Paterson told ABC News.

"It's a device that's filled with fire retardant material that has nozzles all over the engine and when the engine gets too hot ... it will automatically release the fire retardant material over the engine, stopping the fire from occurring.

"All buses run very, very hot, and there can be multiple reasons why a fire will start in the engine bay."

He said there had been 40 fires on buses throughout NSW in the past 12 months.

Police say 30 people rushed from the bus when it caught alight. Three required treatment for smoke inhalation.