A police officer was injured and a youth was stabbed at Christchurch's Christmas in the Park concert on Saturday.
A 15-year-old boy received multiple stab wounds, including three to the face which just missed his eye, in what appears to have been a "racially motivated attack", Inspector Derek Erasmus said.
The police officer was hit in the face while he was trying to stop a fight, receiving five hairline fractures, and spent the night in hospital.
Mr Erasmus said the majority of people at the free outdoor Christmas concert, held at Hagley Park, behaved responsibly, but alcohol later ruined the event.
He said the trouble started an hour before it ended with an influx of drunk youths, who kept police busy breaking fights and tipping out alcohol.
"Preliminary discussions have been held with the (Christchurch City) council to see if this event and other future events in Hagley Park should be alcohol-free," Mr Erasmus said.
"I contrast this with the Band Together concert last month where we had nearly double the crowd, it was twice as long, but there were very few problems.
"The difference was alcohol," he said.
Police faced more problems in the city centre. They had to deal with more fights and drunken behaviour, especially involving drunk youths at the Bus Exchange.
Police arrested 10 people at the concert, but Mr Erasmus said it was the tip of the iceberg as police later ran out of staff to deal with offenders.
NZPA