ACT Policing was called to the Jerrabomberra Creek Bridge shortly after 7am to find a badly-damaged vehicle on the creek bed below.

Police believe the vehicle was travelling west on Lanyon Drive when it left the roadway just before the bridge, clearing the creek and hitting the creek bed on the opposite side.

It came to rest metres from the water.

The male driver freed himself from the wreckage and flagged down a passing motorist for help.

The passenger, the 34-year-old male, was trapped in the vehicle for about 30 minutes before being freed.

He was taken to Canberra Hospital with serious injuries. Last night he remained in a critical condition.

ACT Policing said the driver's injuries were not considered life-threatening.

ACT Policing's Collision Investigation and Reconstruction Team inspected the scene for several hours but a representative said it was too soon to speculate on what could have caused the crash.

Parts of Lanyon Drive were closed at both ends by ACT and NSW police for approximately three hours while investigators examined the site, causing some confusion for motorists.

A Gilmore resident who came across the accident ''basically straight after it happened'' said he was shocked by how serious it looked.

''I couldn't believe anyone got out of it [alive], it looked so bad,'' he said.

Debris was scattered ''about 20 metres'' around the car. ''I was on my way home from work when I saw it,'' he said.

''I called emergency services and the police.''

He believed the two occupants had been on their way to the snow.

Anyone who may have witnessed the incident is urged to contact Crime Stoppers on 1800 333 000.