A witness has described seeing a person holding onto the back of a van shortly before they were found critically injured on Tunnel Road late Saturday night.

The witness told chrislynchmedia.com they saw the person clinging to the rear of the van in Lyttelton while a group of friends nearby appeared increasingly concerned.

The witness said the group began shouting for the person to get off the vehicle as it approached a stop sign.

“They were freaking out and yelling for someone to get the car to follow him,” the witness said.

The witness said they later travelled through the Lyttelton Tunnel and, after reaching the 100 kilometre an hour section of Tunnel Road, came across several vehicles with hazard lights on and a crowd of people gathered on the roadside.

They stopped and asked whether they could help, but were told an ambulance was already on its way.

The witness believed the person may have remained on the vehicle for much of the high speed section before falling from it.

A police spokesperson told chrislynchmedia.com officers were called to Tunnel Road about 11.40pm after a person was found on the road with serious injuries.

Police said initial indications were that the person may have fallen from a moving vehicle.

They were taken to Christchurch Hospital in a critical condition.

The Serious Crash Unit examined the scene and police enquiries are continuing.

A St John spokesperson told chrislynchmedia.com emergency crews were notified at 11.32pm of a motor vehicle incident on Tunnel Road in Heathcote Valley.

Two rapid response units, an operations manager and an ambulance responded.

Ambulance staff treated one patient before transporting them to Christchurch Hospital in a critical condition.