An 18th birthday party descended into an out-of-control brawl between two rival youth gangs that resulted in 20 people requiring hospital treatment including one for two broken arms and a broken leg.

A police spokeswoman said "tension grew" at the party in a workshop at Hobill Ave, Wiri, on Friday night after a group of uninvited people, aligned to a rival youth gang, turned up.

Around 50-100 family and friends had been celebrating the birthday.

At 12.45pm one of the gatecrashers was punched by one of the guests resulting in "punches being thrown" by a number of people, the spokeswoman said.

"A huge brawl then began and weapons of opportunity were grabbed from the workshop and used including wrenches, lengths of timber, metal chair legs, metal bars and bottles."

The spokeswoman said six people were taken to Middlemore Hospital by ambulance with a variety of injuries and a further 14 party goers were treated there after being taken by car.

The most severely injured suffered two broken arms and a broken leg.

Police hope to speak with him once he recovers from surgery.

No arrests have been made.

Meanwhile, an uninvited guest to a party in Bleakhouse Rd, Howick, on Friday is being hunted by police after he ''king hit'' a fellow party goer who is now in an induced coma at Middlemore Hospital.

The 18-year-old was knocked unconscious and hit his head on the concrete after trying to eject the man and other uninvited youths.

He suffered a brain bleed and is now recovering in Middlemore Hospital.

Police are still interviewing party goers about the assault.

In a third attack over the weekend a 22-year-old man was found lying on the road in Otara after leaving a family gathering on Rongomai Rd on Saturday morning.

The man left the address alone at 2am and was later found by family members near the intersection of Rongomai and Dawson roads.

The spokeswoman said the man had suffered serious facial injuries and was believed to have been assaulted.

- © Fairfax NZ News