Police say an attack on a 18-year-old man in Auckland was cowardly, unprovoked and could have killed him.

They have released CCTV footage of an alleged offender sitting on the front of a Nissan Pulsar before the assault on Elliott Street at about 3.30am on September 4.

He knocked the victim to the ground with one punch to the face.

The victim lost consciousness and has a fractured skull and concussion.

"This was an unprovoked attack that could have easily resulted in the death of the victim," says Detective Senior Sergeant Aaron Pascoe.

The offender, a 1.82m-tall male Polynesian of medium to athletic build, aged in his early 20s, was wearing a long sleeved red checked shirt.

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.