An electrical fault is likely to have started a fire in the engine bay of a bus that burst into flames on the Sydney Harbour Bridge and caused traffic chaos in the city during rush hour last week, safety investigators say.
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An interim report by the Office of Transport Safety Investigations has also recommended a full inquiry into the fire to determine whether it might have been avoided, and advise on ways to improve the safety of NSW's buses.
While retro-fitting of government-run buses with fire suppression systems is due to be completed by the end of this year, a multi-million-dollar contract to do the same with 2400 privately operated buses has yet to be awarded.
Transport Minister Andrew Constance said he had formally written to Transport for NSW at the weekend to "make it clear to them that everything must be done to expedite the retro-fitting of the systems on the private bus network".
"We set a two-year goal. We are on track for that but we are going to see if we can better it," he said on Tuesday.
"It isn't just a simple case of turning up at a depot and retro-fitting – we have got to continue to run a transport network and at the same time we have got 27 different bus types that need to be retro-fitted.
"We are not going to rush it and then have the systems fail."
An interim report by OTSI, released on Tuesday, said early indications were that the fire began within the engine bay at the rear of the bus and the "initiation may have been electrical in nature".
"Once the fire began in the engine bay, the fire was fuelled by hydraulic oil reservoirs and diesel fuel lines," it said.
The bus was one of about 265 in State Transit's 2700-strong fleet that still need to have fire suppression systems installed by the government's deadline of the end of the year.
The systems detect a fire or heat within a bus's engine bay, before activating a high-pressure mist.
The report said some of the passengers seated towards the back of the bus saw smoke rising from the vehicle's rear and warned the driver about 5.22pm.
The driver stopped the bus near the north-western pylon of the harbour bridge, opening the rear and front doors and told the passengers to get off.
The 22 passengers on the bus from the CBD to Northbridge were evacuated in less than 15 seconds, before the fire spread rapidly from the engine bay to the vehicle's interior.
The report also noted that another State Transit bus whisked by on the bridge despite the driver trying to stop it as he attempted to warn motorists of the impending fire.
The driver and two passengers were later taken to hospital suffering from smoke inhalation.
The 20-year-old bus was not fitted with a fire suppression system but did have a fire sensor in its engine bay to warn the driver of possible fire or a higher temperature. The sensor activates a warning light on the driver's dashboard.
It was the 36th bus fire in NSW so far this year, raising the prospect that a spike in incidents recorded in 2015 will be surpassed.
Mr Constance said OTSI had previously found the causes and location of bus fires varied "so there is no systemic reason for what is the cause of fires on buses".
But Labor transport spokeswoman Jodi McKay said she was disappointed at the government's response over the past year because the latest incident had not been a one-off.
"We are seeing an increase in the number of bus fires and the minister has been deficient in not fast tracking this [installation of fire suppression systems]," she said.
The state recorded 40 bus fires last year, compared with 29 in 2014 and 28 a year earlier. Of the incidents last year, 11 were electrical fires, the majority of which were caused by short circuits.
OTSI noted in a report last year that the electrical fires highlighted the need for "extra vigilance in checking the condition and securing of electrical wiring".
