Senior Sergeant Anthony Bond, of Dunedin, said officers were called to the Mornington Tavern about 11.30pm on Saturday.
A 56-year-old man had been drinking with some friends when a fight broke out, Snr Sgt Bond said.
A 21-year-old man working as a security guard tried to break it up.
The group fell over, landing on the guard and dislocating his knee.
The 56-year-old man was followed outside by an unknown man who confronted him before knocking him unconscious.
Both the 21-year-old and 56-year-old were taken to Dunedin Hospital for assessment.
Police wanted to speak to the unidentified man or any witnesses.
It was unclear what started the fight, Snr Sgt Bond said.
■ Contact police on 105 and quote file number 230319/ 0669