An 18-year-old man is in a serious condition in hospital after being trapped in shallow water when the vehicle he was a passenger in was involved in a collision in the Blue Mountains.

He was released by emergency services after an hour. The silver Hyundai in which he was travelling was involved in a collision with a blue Ford Fiesta on Orient St, Katoomba, at about 9pm on Sunday.

The Hyundai rolled down an embankment filled with shallow water on its roof when the passenger was ejected and pinned underneath the car in the water.

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He is now in Westmead Hospital in a serious but stable condition.

The 22-year-old female driver of the Hyundai was taken to Katoomba Hospital for mandatory testing but was uninjured, as was a 16-year-old male passenger.

Five teens in the Ford, aged between 16 and 19, were also unhurt.

Investigations are ongoing and anyone with information is urged to contact Crime Stoppers.