Referee Brandon Hale penalised Crescent after the player barged into a Strath Taieri player who was picking up the ball to take a penalty.
The referee brought out the offending player and the Crescent captain. After briefly speaking to the two players, the referee showed the offending player a red card for his actions.
As soon as the red card was shown the dismissed player threw a punch at the referee. The punch hit the shoulder of the referee and then his head.
A melee then eventuated which lasted for a few seconds before calmness was restored.
The game was then called off with Strath Taieri the winner by 29-24.
An after-match function took place, although the player red-carded did not attend. He had been transported away from the ground.
Crescent club captain Allan Thompson declined to comment when contacted yesterday, saying the matter was now part of a union judicial process.
Otago Rugby Football Union general manager Richard Kinley said he could not comment as the incident was the subject of a judicial process.
But the union was giving the referee plenty of support along with the national union and the Otago Rugby Referees Association, Kinley said.
Strath Taieri president Shane Taylor said the game had been played in good spirits and many were stunned when the punch was thrown.
He said it was not a great finish to what had been a great game.
Police confirmed yesterday it was investigating a complaint of assault following a rugby match in Middlemarch on Saturday.
While the investigation was ongoing police were unable to comment further.
The referee was not physically injured but was badly shaken by the incident, Otago Rugby Referees Association chairman Chris Hart said.
He described the incident as deeply concerning.
"To have this happen to a guy who has given up his Saturday afternoon, to go out and referee a game, it is gutting,'' Hart said.
"If he does not get life [a life ban] then we will feel he will get away with it. To me it is right up there. You just can not touch and hit a referee.''
Hart said referees worked hard but too often abuse was let go and that just snowballed, ending up with incidents such as the one on Saturday.