A bus caught fire or reported “excessive heat” and smoke every four days, on average, in NSW last year.

There were 21 bus fires and 69 “thermal incidents” in 2017, up 16 per cent compared to 2016, an Office of Transport Safety Investigations report into all buses and coaches in the state found.

The number of buses with such problems has increased each year since 2013, from 28 in that year to 90 in 2017.

The increase “appears to be a result of a combination of increased reporting by operators and a rise in thermal incidents”, the report found.

The incidents affected 865 passengers, with a school bus holding 70 passengers the busiest to be stopped due to overheating or fire.

There were no injuries reported in 2017 and fewer buses caught fire (21) than in 2016 when 37 burst into flames.

The five buses that were destroyed by fire in 2017 were built between 1995 and 2004. Two buses suffered major damage, 20 sustained minor damage, 60 were “smoke damaged”, and three reported smoke but were not damaged.

Engine bay fire suppression systems (EBFS) were fitted to all NSW public transport buses by September 2017, but only 63 of the 90 buses that caught fire or overheated in 2017 had an EBFS system fitted.

There were “significant disruptions” to the transport network as a result of some fires, investigators found, but in 43 per cent of cases the bus involved was not carrying passengers.

Bus drivers were the first to either see or smell smoke or flames in 73 per cent of the mishaps, while a passing motorist raised the alarm in five per cent of cases, pedestrians informed the driver three per cent of the time and passengers told the driver in two per cent of cases.

Fire suppression systems were effective in several fires, including one that started in the engine bay of a gas-powered bus, with all passengers evacuated safely.

“The fire flashed over into the passenger saloon and the damage was such that the bus was considered unrepairable,” the report said.

Another fire that started in the engine bay of a gas-powered bus was put out by the fire suppression system, despite the driver not shutting off the gas supply.

Another fire that was reported started in the engine bay of a bus as it was returning to the depot after having the EBFS system fitted. There were no passengers on-board and the suppression system activated but the fire reignited after the bus arrived at the depot.

A Transport for NSW spokesman said they are continuing work to improve bus safety via a range of measures including a review of Australian design rules, improvements to maintenance processes and regulatory oversight, as well as a review of bus fire safety in rural and regional buses.