State Transit says an electrical fault is thought to be behind a bus fire in Sydney last night as public transport safety in Australia's biggest city continues to come under the microscope.

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The engine compartment of the route 440 bus caught fire on George Street near Central Station about 9pm.

No passengers were injured in the incident.

It was the third major bus fire in Sydney since September, when a bus caught fire on Sydney Harbour Bridge, causing traffic chaos.

That incident prompted the Rail, Tram and Bus Union to call on Transport Minister Andrew Constance to resign.

Earlier this month, two buses from the long-term car park at Sydney Airport were gutted by fire, with authorities claiming one of the buses had overheated.

"The driver was able to evacuate all passengers, so there was no safety issue. They were all evacuated without injury," State Transit CEO Peter Rowley said of the latest incident.

"The fire suppression system which is on this bus activated successfully ... that allowed for emergency services to extinguish the fire as soon as they arrived about a minute after."

Mr Rowley said the bus which caught fire last night had a major service, and two safety inspections in the past four months.

He said the bus was "one of the newer" vehicles in State Transit's fleet.