The man was admitted to Dunedin Hospital after an alleged unprovoked attack in an alley off Frederick St, near the Bowling Green Tavern, in Dunedin, about 1am.
He spent the day in surgery on Monday, as surgeons operated on a serious head injury he received when he was allegedly kicked and fell backwards.
He had gone in to the alley to be sick when police allege the attack happened.
Detective Sergeant Chris Henderson said the teenager was stable in the intensive care unit yesterday, but was still in a "very bad way".
There were a lot of people in the area at the time and people who saw the incident may not have realised the seriousness of what had happened, he said.
A 21-year-old man, arrested later that day at another Dunedin address, is in custody and charged with injuring with intent to injure.
He appeared in the Dunedin District Court on Monday, but was remanded until Friday while police continued their investigation.
Police had spoken to some witnesses, but needed to corroborate the information they had, Det Sgt Henderson said.
They were particularly interested in speaking with a fair-haired girl wearing a blue dress and holding a pair of black shoes, who ran up to a security guard in a car stopped at lights at the intersection of Frederick St and State Highway 1, and told him someone had been "killed" in the alley.
She pointed out the man allegedly involved to the security guard, but then disappeared.
Anyone with any information should contact Det Sgt Henderson on 471 4800.
