The assault occurred about 3.20am on Tuesday in Cow Lane, and was sparked by an altercation in a bar in Camp St between two pairs of men.
They were all kicked out of the bar.
About 20 minutes later, one of the alleged offenders came across one of the other men outside.
"He's tried to confront him, and the victim's just pushed him away," Watt said.
The offender then punched the victim in the head.
"He's then hit the ground and hit his head on the ground."
The alleged offender then ran off.
The victim, who suffered a fractured skull, was taken by ambulance to Lakes District Hospital in a serious condition, and later flown to Dunedin.
The incident was captured on CCTV, and Watt said police were appealing for information.