A group of drunk mostly female teenagers attacked a police officer who had been attempting to stop an assault on one of the girls in the group.
The sergeant will need to undergo surgery to fix a broken nose during the incident in Christchurch last night, Senior Sergeant Corrie Parnell said.
He also suffered cuts to his face and ear after being set upon by the teens at a party in Riccarton.
The incident happened when police attended a party at a house on Riccarton Road about 10.30pm, Mr Parnell said.
"Police had dealt with the issues at the party and were preparing to leave when the sergeant saw a male assaulting a teenage girl.