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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.
Sgt Corlet said the alleged incident unfolded when the man threw a water bottle towards Kingan after the final whistle.
As Kingan walked towards the changing rooms the man followed, then approached Kingan and punched him once in the face, Sgt 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, Sgt Corlet said.
He 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 alleged incident.
"I'm not going to deny it happened but I actually had my back to it," he said.
Umbers did not know 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 alleged incident but said it was not appropriate to say too much.
"I saw it happen, and it will be dealt with," Petrie said.