A 20-year-old Oamaru man faces an assault charge after allegedly punching the referee at the end of a Citizens Shield North Otago rugby semifinal in Oamaru.

Sergeant Blair Corlet, of Oamaru, confirmed the man was spoken to by police last night. He had been served a summons to appear in the Oamaru District Court on Wednesday.

The incident happened after referee Craig Kingan blew the final whistle on the semifinal at Whitestone Contracting Stadium on Saturday.

Valley won 32-29 after extra time in a game that featured three yellow cards to Old Boys players.

Corlet said the alleged incident unfolded when the man threw a water bottle towards Kingan after the final whistle. While Kingan was walking towards the changing rooms the man followed and then approached Kingan and punched him once in the face, Corlet said.

A melee ensued as assistant referees Hamish McKenzie and Roly Senior stepped in to take control. The man was then returned to the group he was with, Corlet said.

Corlet said the man "was a member of the Old Boys club", but was not involved in Saturday's game.

Kingan later appeared shaken by the incident but declined to comment.

North Otago Rugby Union chairman Warren Prescott confirmed the rugby union had launched an investigation into the incident.

North Otago Rugby Referees Association president James Symes declined to comment, but said it "took the shine off two great games of rugby" on Saturday.

Old Boys president Richard Umbers was at the game but did not see the incident.

"I'm not going to deny it happened but I actually had my back to it," he said.

Umbers was not aware if the man at the centre of the incident was a member of the Old Boys club.

Valley president and North Otago refereeing stalwart Tim Petrie witnessed the incident but said it was not appropriate to say too much.

"I saw it happen, and it will be dealt with," Petrie said.