The assault happened outside the Dunedin Social Club in the Octagon around 1.10am, Sergeant Matt Lee, of Dunedin, said.
When police arrived the 19-year-old had regained consciousness and managed to walk to a nearby ambulance.
Witnesses told officers the alleged assault happened when the teenager plucked a sombrero off another bar patron's head.
The attacker punched the teenager in the face and then fled the scene.
Sgt Lee said inquiries were ongoing to identify the offender, including by viewing CCTV in the area.
About 20 minutes later, late-night antics started to kick off at nearby club Surburbia in Lower Stuart St.
At 1.30am, two men were removed from the club due to their level of intoxication.
One of the men, a 28-year-old, allegedly kicked over a security barrier and started to swing punches at the security guards.
When police arrived he attempted to re-enter Surburbia while verbally abusing attending officers.
The man was then arrested by attending officers, Sgt Lee said.
