A police officer was hospitalised and a teenager was stabbed at the Christmas in the Park event in Christchurch on Saturday night.
A 15-year-old youth was stabbed multiple times, including three times in the face.
The police officer suffered five hairline fractures to his jaw and face after he was hit when he tried to break up a fight.
Inspector Derek Erasmus of the Christchurch police says alcohol fuelled problems in the final hour of the Hagley Park event, when there was an influx of drunk youths. There were 10 arrests.
Inspector Erasmus says there was more trouble afterwards, with police dealing with fights and drunken behaviour throughout the central city.
He says talks have already been held with the Christchurch City Council on whether to make future events in Hagley Park alcohol free.
Inspector Erasmus says October's "Band Together" concert at Hagley Park was advertised as alcohol free and people didn't bring alcohol, nor did they arrive drunk at the event.