Police say alcohol-fuelled violence in central Nelson is a problem.
A young man was beaten with a metal bar, following an altercation between two groups after city bars closed at 4am on Saturday.
The offenders are still at large.
Detective Senior Sergeant Wayne McCoy says it's the fourth such attack in recent months, in which alcohol seems to have been a factor.
All were isolated incidents involving different people.
He says the police are taking steps towards a zero tolerance policy in the city.