A Hamilton man was so angry that his son wasn't playing the position he thought he should be, he headbutted the coach.

Jason Noel Root, 45, was labelled a "total disgrace" in the Hamilton District Court today by by Judge Garry Collin, who was unimpressed with Root's actions at a Hamilton school on June 27.

The court heard Root was on the sidelines watching his son and his St Johns College U14A rugby team train at the school field.

Noticing his son was playing at centre instead of second-five, Root abused the coach and threatened to punch him in the head.

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Afterwards, Root realised what he'd done and apologised to his son's teammates.

However, instead of going home, Root spied the coach in the car park and approached him.

He again started yelling and swearing at him and the pair ended up face to face. Root headbutted the victim, hitting him on the left cheek.

The victim put his hand up to try and ward him off, but Root moved closer and punched the coach on the side of his jaw with a closed fist.

Root continued to follow the victim who was backing away, with the pair ending up in the middle of the field.

The pair exchanged punches as the victim tried to get some distance between them.

The melee was eventually broken up when members of the senior rugby team approached them.

Root continued to abuse the victim yelling that he would "finish it off in the carpark".

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When questioned by police, Root admitted getting angry but denied headbutting him.

Root's lawyer Kerry Tustin said her client had been shaken by the incident so much that he "appeared to now have a nervousness in his voice that he had not noticed before".

"So hopefully this is the salutary lesson that he needed to have."

Root got a dressing down by Judge Garry Collin during sentencing on a charge of common assault in the Hamilton District Court this afternoon.

"You are the worst kind of parenting nightmare on the side of a sports field. You are the very type of person that no team and no coach wants to have watch their child play. This is a total disgrace and you should be ashamed of yourself."

The judge said a term of prison was justified however he was saved by pleading guilty, and although he had a history of violence and intimidation, he had not appeared in court since 2004.

"I am glad to be advised that you have been trespassed from the sports field for two years. It is a pity it is not longer," he told Root.

The judge said the victim was still traumatised by what happened and was acting in a voluntary capacity.

"He is there to help your child and other children to learn to play a game which they enjoy playing, a type of game which can be ruined by someone like you. He is now considering quitting coaching because of what you did."

Judge Collin sentenced Root to 80 hours' community work, nine months' intensive supervision which will be judicially monitored and to pay $500 emotional harm reparation to the victim.