That is the burning question transport safety authorities will continue to have to try to answer after yet another bus caught fire on Thursday night, the 53rd bus fire this year.

The latest incident will be investigated by the Office of Transport Safety Investigations, a spokeswoman for bus agency State Transit said.

But the lack of explanation for Sydney's burning buses is becoming more pressing as the numbers of fires increase. The number of buses involved in fire related incidences has increased from 28 in 2013, to 29 in 2014, to 40 in 2015.

Last year's increase prompted the installation of the fire suppression systems in the government-run bus fleet. A suppression system had been installed on the 440 Rozelle to Bronte bus that caught on fire at around 9.15pm on Thursday night near Central Station on George Street.

All passengers on board the bus were safely evacuated with no injuries partly thanks to the fire suppression system fitted to the bus which was activated "successfully", the spokeswoman said.

"It's important to remember the purpose of the fire suppression system is not to prevent fires but to minimise their impact and give people time to evacuate the bus safely," she said.

Stewart Prins, a spokesman for the Rail, Tram and Bus Union said the installation of fire suppression systems should have happened years ago.

"When a gas-powered bus blew up in Sydney in 2011, and the RTBU demanded action, former Premier Barry O'Farrell stood up in parliament and accused bus drivers of thuggery." Mr Prins said.

"We now see who the real thugs were – because the Liberal State Government chose to stick its head in the sand and indulge in union bashing games rather than deal with a genuine public safety scandal."

"It is only the quick-thinking responses of bus drivers, and a healthy dose of good luck, that has prevented lives from being lost from these fires."

As of the 23rd of December, 2,175 privately-run buses still do not have the system installed yet.

According to a Transport of NSW spokesperson, only 225 privately owned buses out of approximately 2,400 have been fitted with the system.

That is in addition to 400 buses in the private fleet which already have the technology.

All 2,100 State Transit buses will be fitted with the fire suppression system by the end of year.

A 2013 report by the Office of Transport Safety Investigations examined incidents between 2005 to 2012 and found that the number of fires was increasing over time.

Thursday night's bus fire was the latest in a string of recent incidents.

In early December, two buses were completely destroyed and a third was severely damaged while being parked at a carpark near Sydney airport.

A city-bound Hillsbus caught alight on the M2 in November and forced the evacuation of 30 passengers, it did not have a fire suppression system.

In September, a bus fire temporary closed all the lanes on the Sydney Harbour Bridge and three people were admitted to hospital.

It too did not have a fire suppression system.