A teenager is in a critical condition after the car he was driving went through a train track barrier and crashed down a bank in Wellington.

A St John spokeswoman said a 17-year-old male was flown to Wellington Hospital by Wellington Westpac rescue helicopter.

Members of the public kept the driver alive until emergency services arrived at the scene on Middleton Rd, Glenside.

Central fire service communications shift manager Mike Wanoa said two fire appliances were sent to the scene just before 4pm.

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"The car was upside down in a small creek."

The Serious Crash Unit was investigating.

Cordons were in place between Middleton Rd and Takapu Rd and Westchester Dr and Middle Rd.

Police advised motorists to use alternative routes between Tawa and Churton Park tonight.

Train services were unaffected by the crash.

Fire crews had to free the injured person from the wreckage, Wanoa said.