A Mongrel Mob member attacked a teenager at a KFC restaurant in Blenheim because he was wearing a blue rugby uniform, a court has heard.

Kahutia Mita, 34, a vineyard worker, punched the 15-year-old in the head, fracturing his nose and breaking one of his teeth.

Police said the high school student walked to the fast food outlet on December 9 with two of his friends after playing touch rugby.

The boy was wearing a blue team uniform and was of a typical height and build for a boy of his age, a police summary of facts said.

Mita, in comparison, was large and tall, and wearing red, the gang's colour, the summary said.

Mita appeared to be agitated when he walked in, and clenched his fists and verbally abused the student for wearing blue.

He then punched the student twice to the head, shouting "that's what you get".

The student was dazed and confused from the punches, and his mouth and nose were bleeding, the summary said.

Mita then left the restaurant.

After the assault, the student had a fractured nose, a broken tooth that required surgery, a hematoma on each eye, bruises and a concussion.

When spoken to by police, Mita declined to comment.

He was charged with injuring with intent to injure and taken into police custody.

He admitted the charge at the Blenheim District Court on Monday.

Judge Jan Kelly convicted him and remanded him in custody to January 24 so his home address could be assessed for an electronically monitored sentence.