A 25-year-old Alexandra man is in a critical condition in Dunedin Hospital after falling from a van travelling at 100kmh, police say.

The man received serious head injuries when he fell from the van while it was on State Highway 8 between Clyde and Cromwell on Saturday evening.

Senior Sergeant Brian Benn said a ''van load of guys'' were on a ''joy ride'' from Alexandra to Cromwell and back.

''In the Cromwell Gorge the driver has heard a yell as the guy has climbed from the back window of the van and fallen from the van,'' Snr Sgt Benn said.

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''The driver then saw the man rolling down the road, in his rear view mirror.

''[It] seems like boys having fun gone bad, which is sad for everybody involved.''

Police were alerted just after 9pm by ambulance services, who took the man to Dunstan Hospital before he was airlifted to Dunedin Hospital.

Fire crews also attended the incident.

Victim Support were helping the man's family.

Snr Sgt Benn yesterday said there was no indication the driver of the van was facing charges relating to the incident.

Yesterday, in response to a social media post by the Otago Daily Times about the accident, people expressed their support for the man.

One post read: ''Stay strong bro your my best friend I love you mate''.

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Other users said they were thinking of his family and wished him a swift recovery.

A hospital spokeswoman last night said the man was in an induced coma, in a serious condition under intensive care.

Today his condition was given as critical.

An investigation into the incident continues.