A man who took part in a vicious attack on a teenage boy at Christchurch's Christmas in the Park celebration has been ordered to pay $500 to the victim and to do 60 hours of community work.

The 15-year-old ended up with 31 stitches and bruising after the bashing, Christchurch District Court was told today.

Zebadee Jeffrey Sinnott, 18 unemployed, admitted assault.

Police prosecutor Sergeant John Taylor said a drunken Sinnott was with two friends at the event on November 27 when he and the victim bumped shoulders and there was an exchange of words.

Sinnott punched him once and made no attempt to stop a friend then repeatedly punching the victim while holding a glass bottle.

Defence counsel Ros Burnside told the court Sinnott accepted he retaliated wrongly when the victim bumped him. He delivered one punch but his co-offender then stepped in and did the rest of the assault with repeated punching and using the bottle.

"He (Sinnott) is very remorseful," said Ms Burnside. "He has thought long and hard about how it all unfolded. He feels very silly, and stupid and upset with himself. He has thought a lot about the victim and how his actions have led to harm for a young boy."

Judge Michael Radford described Sinnott's actions as "disgraceful behaviour".

Others involved in the incident are being dealt with separately.