The defendants had been cruising around the city in the early hours, having consumed alcohol and MDMA.
In George St, they saw a member of the public confronting the victim and yelling about his criminal convictions.
The group - including Chance John Edwards, 20, Caleb Edward Ross Stefani, 19, and Matthew Robert Johnston, 21, - searched online to verify the claims.
In 2013, they found, the man had been jailed for nearly five years for sexually abusing a young girl.
Despite having no connection to the case, the men parked on Moray Pl, exited the car and launched into the vigilante violence.
They surrounded the victim. Murphy-Fox was first to strike. He punched the man in the head then held him for Edwards to do the same.
Kicks from Murphy-Fox sent the victim to the ground but gave him the opportunity to break free.
However, Stefani pounced, aiming a punch at the man's head but hitting his arm, before leaving the scene.
Johnston threw a bottle at the victim's head, which missed.
Murphy-Fox and Edwards then beat the man in tandem but it was the former who took the violence up a notch.
He held the victim from behind in a "bear hug" then dropped him on to the road head first, knocking him unconscious.
While he lay there, Murphy-Fox kicked him in the head.
"What did you two brave soldiers do then?" Judge Phillips asked.
"You ran away ... from a man needing urgent medical attention."
The victim suffered concussion, a laceration to his head and a fractured neck.
The court heard he was re-hospitalised when he suffered seizures as a result of the beating.
"It was unprovoked, gratuitous street violence," the judge said.
Johnston was the only member of the group with a significant conviction; he had been jailed for an armed robbery in Oamaru when he was a teenager.
Judge Phillips sentenced him to seven months' home detention and 270 hours' community work.
Edwards received seven months' home detention and 190 hours' community work; and Stefani got 220 hours' community work and 18 months' intensive supervision.
All but Murphy-Fox were ordered to pay the victim $500.