Why are Sydney's buses catching alight?

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.