He said he didn't see the attack coming.
"I was walking off the field back towards the guys waiting on the sideline and I turned and there it came."
He said the punch was deflected a bit by his arm, but it still caught him on the left side of his head.
"I took one little glance, but then scurried off the field as fast as I could."
He said the assault hadn't put him off refereeing.
"I'll be back out there. It was an experience and something I'll be wary of, but I'm pretty passionate about refereeing so I'll be keen to keep going."
Angela Fahey, whose son played for Auckland Grammar, said the behaviour of the De La Salle players deteriorated in the final 10 minutes of the game.
"The spectators were shouting abuse which also helped incite what we could see was heading to a very bad outcome."
As soon as the final whistle blew, the De La Salle team "raced" towards the other team.
"My boy was hit twice in the face."
Spectators also took to the field, she said.
Ms Fahey said she saw the attack on the referee and said it came from a player. De La Salle players fought each other as some tried to stop the fight.
Witness Brett Arthur, who is also a rugby referee, said he had never seen such behaviour at a match.
He said about 50 parents and supporters of the De La Salle team charged the pitch after the final whistle.
"They were pelting them [referees and Auckland Grammar players] with bottles and anything they could get their hands on. It was like nothing that I'd ever seen before."
De La Salle principal Myles Hogarty said the school was conducting a thorough investigation.
Auckland Rugby Referees' Association chairman Don Crawford said he had spoken to both schools yesterday but would not comment further until the investigation was finished.
Auckland Grammar School principal John Morris said his team's coach, Mark Vella, did a great job in separating the players from the violence.