The victim suffered a fractured jaw, suspected fractured ribs, and bruises and grazes.
He was taken to Dunedin Hospital where he underwent surgery.
Mr Hanna said the boy's injuries were not life-threatening but were serious.
"This was a cowardly unprovoked group attack on this young man and he has received serious injuries as a result," he said.
"It is believed that there was a party taking place nearby and there were a large amount of people in the vicinity of where the attack took place.
"At this stage the identities of the people involved in the assault have yet to be confirmed, however police are following a positive line of inquiry.
"We are interested in hearing from those who attended this party and also who witnessed this assault."
Anyone who has any information on the assault was asked to contact the Dunedin police on 03 4714800 or call Crimestoppers, anonymously, on 0800 555 111.