Auckland police say the off-duty officer left in a serious condition after trying to stop a fight was subjected to a sustained attack by more than one person.
The officer, 39, tried to stop a fight between two teenage girls in Tuakau, in Franklin District, on Friday night.
He has a fractured skull, a punctured lung, serious facial injuries, a broken jaw, a broken ankle and missing teeth.
Police say he was set upon by youths who had been watching the fight, and later collapsed.
Superintendent Mike Bush says a number of people who were at the scene knew the man as an off-duty police officer.
"Our off-duty officer was set upon by at least one person initially, from behind, knocked to the ground and at that stage received a beating from more than one person and including a number of kicks and possibly stomps."
He says the officer is in the High Dependency Unit at Middlemore Hospital, and has regained consciousness.
Superintendent Bush says police have not yet arrested anyone but have credible information about who the attackers are.
Worrying precedent - Association
The Police Association says serious assaults on two officers over the past 24 hours sets a worrying precedent in that if officers are vulnerable, so is the general public.
As well as the Tuakau incident, a Whangarei officer has serious facial injuries after an assault by a suspected drunk driver on Friday night.
The association says these violent acts towards police are concerning as officers are the last line of defence for law and order.
President Greg O'Connor says while these assaults are an issue for individual officers, they should be an even bigger concern for everyday New Zealanders.