The court was told the confrontation between members of the Tribesmen and the Bloods, or Thirteen BZ, gangs was sparked by a threat to stab one of Brack's nephews.
Judge Grant Fraser said the "outrageous behaviour" stopped only when members of the public pulled the two sides apart.
"What happened on that day I can only describe as an unbelievable sequence of events, where you and a number of others were totally out of control," he said to Brack. "Innocent members of the public were caught up as well ... I suspect for them it was absolutely terrifying. I don't know if there's any understanding on your part of the impact on other people."
The brawl was "vigilante behaviour at its worst" and would not be tolerated because it put innocent people at risk.
However, Judge Fraser also noted Brack's background and that most adult members of his whanau were involved in gangs and in jail. He had no doubt that background had shaped Brack's behaviour - he had two previous identical convictions - but that was no excuse.
Judge Fraser also noted that Brack had managed to gain qualifications in automotive welding. He urged him to step away from gangs or risk spending the rest of his life in and out of jail. After discounts for his age, early guilty plea and some remorse, he was sentenced to eight-and-a-half months' jail. Brack has been in custody since October so is likely to be freed soon. He denied being a prospect for the gang, as police claim.
Those already in prison for taking part in the brawl include Grace Brack, jailed in December for six months for aiming her car at two people fighting in the road before swerving and hitting a third person. She had three young children in the car at the time. The longest sentence, 23 months, was handed down to Levi Reeves. Two people have pleaded not guilty and will go on trial later this year.