Call it a burning desire to get the correct fare.

Transport investigators examining the bus fire that shut down the Sydney Harbour Bridge last September were left flabbergasted upon discovering a passenger ignored flames to jump back on to the bus to "tap off" their Opal card.

The fastidious passenger, Fairfax Media has learnt, was one of 22 people evacuated from State Transit service travelling northbound on the bridge before the driver noticed smoke spewing from the engine compartment into the back of the bus.

An initial report by the Office of Transport Safety Investigations found that passengers were evacuated within 15 seconds once the driver stopped near the north-western pylon as evening peak-hour traffic piled up behind.