Intelligence analyst Constable Sean Drader said the incident occurred about 1.30am in The Mall and a 31-year-old man was left unconscious.
The incident began when the offender "shoulder-barged" the victim "basically looking for a fight".
"He [the victim] ignored him and walked past him. The offender ran up behind him and punched him in the back of the head.
"That knocked him over and he hit his forehead and was knocked unconscious."
The victim was taken to Lakes District Hospital and treated for a 3cm gash on his hairline.
The incident was witnessed by a bouncer at Chico's who attempted to hold the offender.
However, the man ran down The Mall and then Cow Lane, Const Drader said.
The offender is described as being European with blond dreadlocks, about 1.8m tall and wearing a white T-shirt and dark beanie, which is not visible in the image taken from CCTV cameras in Cow Lane.
Anyone with information was asked to contact Queenstown police on (03) 441-1600.
