A teenage girl has been taken to hospital after falling from a moving double-decker B-Line bus on Sydney's northern beaches. This was reported by Qazaqyia.kz citing The Guardian.

Emergency services were called to Manly Road at Seaforth on Monday evening after the teenager fell from the B-Line bus near the Spit Bridge. She was treated at the scene for head and hip injuries and taken to Royal North Shore hospital, where a spokesperson confirmed on Tuesday she remained in a stable condition.

A Transport for NSW spokesperson said it was aware of the incident on Spit Road and that the bus had been withdrawn from service while an investigation by bus operator Keolis Northern Beaches and the department takes place.

"The passenger was treated at the scene and taken to hospital with minor injuries," they said. "We wish the passenger all the best in her recovery."

The incident has also been reported to the Office of Transport Safety Investigations.

On Tuesday, the Rail, Tram and Bus Union (RTBU) said Keolis Northern Beaches "must urgently explain how a door on one of its B-Line buses reportedly failed" and that it was investigating reports from drivers that doors on the fleet had opened unexpectedly in the past.

RTBU NSW Tram and Bus Division secretary Peter Grech said private operator Keolis and Transport for NSW must "come clean" about whether warnings had been ignored.

"A 16-year-old girl fell out of a moving bus … on to one of the Northern Beaches' busiest roads. It is horrifying to think how much worse this could have been," Grech said. "She should have been safe on that bus and this was undoubtedly a highly traumatic experience for her.

"Drivers are telling us this may not be the first time one of these doors has opened unexpectedly. We are currently checking the circumstances, but Keolis cannot leave passengers wondering whether there were warnings it simply failed to act on.

"If a door can open while a bus is moving, then that bus cannot be carrying the public," he said.

Keolis responded to Guardian Australia's request for comment on Tuesday afternoon with a statement confirming it was "aware of an incident that occurred last night on our B-Line service which resulted in a passenger falling from the bus near the Spit Bridge at Seaforth".

"Our immediate thoughts are with the passenger and her family and wishing for a swift recovery."

It is understood Bustech double-decker B-Line buses have been taken out of service, although Keolis and Transport for NSW have not confirmed which vehicles were withdrawn or when.

It has not been established when and how earlier concerns were reported to Keolis management, the response drivers received or what steps were taken to control the risk.

It is understood the union is seeking records from Keolis and Transport for NSW under the work health and safety act as part of its investigation.