A police sergeant in Whangarei needed hospital treatment after being struck by a rock while trying to arrest two men outside a suburban party.
Police say onlookers started throwing rocks and bottles at the officers, outside the party on Otaika road, in Raumanga.
Sixteen extra officers were called in to bring matters under control.
The sergeant was treated for concussion and received 15 stitches to his head. He was later discharged from hospital.
Six people face charges for fighting and obstruction.
