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.
"He immediately intervened but the offender resisted arrest - and the girl, who was the subject of the assault, began attacking the officer, punching and kneeing him in the head.
"She was joined by several other girls who continued the attack until other officers could come to the sergeant's aid."
Two partygoers were arrested at the scene, Mr Parnell said.
A 17-year-old male has been charged with assaulting a female, resisting arrest, and assault a police officer, and will appear in court on Tuesday.
A 14-year-old girl has been charged with assault with intent to injure, and will appear in Youth Court.
The officer has been stood down from duty and was recovering at home, Mr Parnell said.
"This kind of attack on officers who are trying to protect victims and members of the public is absolutely senseless," he said.
"Not surprisingly, alcohol was a major factor.
"Alcohol is often behind this sort of violence and disorder and in this incident most of those involved were showing high levels of intoxication."