The fracas started when a drunken reveller threw a punch, sparking a brawl that stretched 100m up the street.
The youth, who knew the injured teenager, said he was felled by a single blow.
"He got punched in the head and fell to the ground."
The youth said he was left shaking uncontrollably but few knew the extent of his injury because the attack had happened about 100m from the main brawl.
The youth said many teenagers at the party including the victim went to school at nearby Tamaki College.
Emergency services were called when another teen collapsed after drinking too much alcohol, he said.
Most people who had spilled out of the house were unaware the schoolboy had been seriously hurt further along the street.
Neighbours near the house where the party took place said teenagers started gathering from 6pm.
The fighting suddenly erupted around 10.30pm and went on for half an hour.
Beer cans and plastic cups were thrown during the incident.
Neighbours said they could hearing screaming and insults as the teens traded blows.
"They were just drunk," said the youth.
"They were talking smack and just wanting a fight."
Police got to the address and found the boy injured just before 11.30pm.
A police spokesman said the 15-year-old was rushed to hospital by ambulance.
"He has head injuries and is in an induced coma," he said.
This morning police have been at the scene speaking to residents and partygoers.
"If anyone saw the disorder and has information that would help investigators, police would appreciate their assistance," a police spokesman said.
Can you help?
If you have information about the incident contact the Glen Innes police on 09 526 7300, or Crimestoppers anonymously on 0800 555 111.
- Additional reporting Anna Leask
