Witnesses were still trying to administer first aid.
Mr Glossop said several knives had been found in the vicinity but the murder weapon had not yet been identified.
Police were following strong leads in finding the person responsible, he told One News.
The body of the youth, who was a mechanic and a car enthusiast, had been removed from the scene this afternoon and was being formally identified.
Police were talking to partygoers about what happened.
"Police have notified the deceased's immediate family but were giving them time to advise other family members and need to conduct a formal identification before his name can be released," Mr Glossop said.
- NZPA