"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."