A 16-year-old girl pinned under a bus for more than 90 minutes in Parramatta last night is in a "serious but stable" condition in hospital.

Key points: The three teenagers were 16 years old, two sustained minor injuries

The three teenagers were 16 years old, two sustained minor injuries It is believed about 20 passengers were on board the bus at the time of the collision

It is believed about 20 passengers were on board the bus at the time of the collision The 62-year-old bus driver was taken to hospital for shock and mandatory blood tests

The Sydney public transport bus mounted a footpath about 9:45pm last night and hit three teenage girls who were waiting at the bus stop at the intersection of Church and Argyle streets, outside the Parramatta train station.

One of the girls was pinned beneath the front of the bus.

Paramedics, police and firefighters worked frantically for one-and-a-half hours to free her.

Rescues crews were seen huddling around the bus, its front windshield cracked, parallel against a wall.

The bus was caught beneath an awning, meaning it was difficult for firefighters to elevate the bus and free the teenager.

"On our arrival there was a female in her early teens that was trapped under the bus for a significant period of time," NSW Ambulance Acting Inspector Joe Ibrahim said.

"She was crushed by the bus and she remains in an extremely critical condition and has been transported to Westmead Hospital with pelvic injuries.

"The rescue was extremely difficult … with the assistance of NSW Fire and Rescue and NSW Police Force we managed to get the bus off the female patient and had our medical teams put her to sleep."

Emergency services described freeing the teenager from under the bus as a "very arduous task". ( ABC News )

Mr Ibrahim said the other two teenage girls, also 16, sustained minor injuries and were "thankfully, all quite stable".

The bus had about 20 passengers on board at the time of the collision and a 49-year-old female passenger was treated in hospital for an eye injury.

Inspector Phil Sheedy from Fire and Rescue NSW said the confined space between the bus and the wall made the rescue "a very arduous task".

"Hopefully, she will pull through," he said of the 16-year-old girl.

NSW Police said a 62-year-old male bus driver was taken to the hospital for shock and mandatory blood tests as investigations continued.