It happened near Lancaster St as the 17-year old was walking along York Ave toward Wharerangi Rd.
He was listening to music with headphones and was unaware of anyone approaching when he was "inexplicably punched to the head from behind," police said.
Police are interested in speaking to a group of other young men who may have witnessed the attack.
The victim told police a group of five was nearby but he could not say if they had any link with the attacker.
The offender was described as aged about 17, with brown skin and medium-length hair.
Police were also still seeking the man involved in the Taradale attack, which happened at about 2.40pm on Tuesday night between the Pettigrew-Green Arena and Murphy Rd.
The man was thought to be in his 40s or 50s.
Police said anyone who may be able to assist with inquiries should phone Hawke's Bay police on 06 837 0100 or the Crime Stoppers line.