An unprovoked attack in Auckland's CBD could have easily left an 18-year-old dead, police say.

The young man was punched in the face about 3.30am on September 4, after the offender approached him on Elliott St.

"The punch knocked the victim to the ground where he lost consciousness.

"The victim's injuries included a fractured skull and concussion," a statement from police said.

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The offender is described as a male Polynesian, 6 feet tall, medium to athletic build, aged in his early 20s, and wearing a long sleeved red checked shirt.

He was caught on CCTV shortly before the attack, sitting on the front of a Nissan Pulsar.

"This was an unprovoked attack that could have easily resulted in the death of the victim," Detective senior sergeant Aaron Pascoe said.

Anyone with information about the identity of the man, or any other information should contact Detective Alicia Bergman at Auckland central police station on 09 302 6400.