The fire was extinguished about 3pm by two Fire and Rescue NSW crews, with assistance from airport fire services using bulk foam pumps.

The buses were empty when the fire started, with no persons or other parked cars nearby, Fire and Rescue NSW said.

Shortly before the fire, the driver of one of the buses had left the car park to report a mechanical fault and overheating of the bus, a spokesperson for Fire and Rescue NSW said.

By the time the driver had returned, the bus had caught alight.

The fire spread and burnt out a second bus parked next to it, and severely damaged a third.

The exact cause of the fire has not yet been determined, due to the severe damage to the buses.

Thick black smoke was visible in the area surrounding the carpark, prompting many emergency calls to Fire and Rescue NSW.

Sydney Airport has continued to operate as normal and flight departures have not been affected, a spokesperson for the airport said.

The incident follows several incidents of bus fires across Sydney in the past months.

In September, three people were admitted to hospital after a bus full of peak-hour commuters caught fire at Milsons Point, closing all lanes of the Sydney Harbour Bridge for several hours.

Initial investigations suggested the incident was most likely due to an electrical fault in the engine, which escalated because the bus did not have a fire suppression system installed.

Last month, a city-bound Hillsbus bus caught fire on the M2 in Beecroft, requiring the evacuation of 30 passengers.

The bus also did not have a fire suppression system installed.

Fittings of the fire suppression systems in public buses are expected to be finished this year, but private bus fittings will not be completed until August next year, Transport for NSW said.