Steven Churchis, 18, attacked Edwin Linder with a "martial arts-style kick" then punched and stomped on his head while he lay on the ground.
Mr Linder, 42, lay there for eight hours before a member of the public called an ambulance.
He died in hospital three days after the assault in August last year.
In July, Churchis was found guilty of murder following a trial in the High Court at Auckland and also admitted a charge of wounding with intent to injure from earlier on the night of the incident.
Justice Geoffrey Venning imposed a mandatory life sentence and a minimum period of imprisonment which took into account the offender's youth and remorse, offset by the savage nature of the attack.
On the evening of July 30, Churchis - who was living on the street at the time - was drinking and smoking synthetic cannabis with friends in Albert Park.
When he believed a member of the group was paying too much attention to his girlfriend he threw a half-drunk bottle of beer at the man's head before hitting him with a flurry of punches.
The man ran off, his face covered in blood.
Hours later Churchis' violent rampage came to a head when he found his two friends in Mills Lane, arguing with Mr Linder who they had found defecating in the spot where they had been sleeping.
Claiming he was acting in self-defence, Churchis attacked the homeless man who had significant disabilities from a previous head injury.
The trio left him on the ground and went to a park to sleep.
Six hours later they returned to find Mr Linder unconscious and barely breathing.
Justice Venning said Churchis took the opportunity to warn his friend to keep quiet, saying something along the lines of: "I'll beat you to a pulp twice as worse as the other two".
Crown prosecutor John Dixon said Mr Linder was a "particularly vulnerable victim" and the seriousness of the teen's offending was increased by the fact he did not call an ambulance.
Churchis' lawyer Peter Winter said a psychiatric report showed his client was experiencing flashbacks at the time, which had sparked the brutal assault.
He said the report clearly showed that seeing recent attacks on his mother by her partner had traumatised the young man.
Churchis sat in the dock with his head bowed throughout today's sentencing.
- By Rob Kidd of APNZ
