The gallery cheered as Justice Quinlan handed down the sentences, while Cassius’ mother Mechelle Turvey burst into tears, local media reported.
At trial, Brearley, 24, and Palmer, 30, had each blamed the other for Cassius’ death.
Justice Quinlan found Brearley delivered the fatal blows, adding that he had shown “no remorse whatsoever”.
Brearley had alleged that he acted in self-defence as Cassius was armed with a knife, which the judge rejected as “complete fabrication”.
“You cannot make amends when you don’t acknowledge the pain that you have caused.
“You cannot be remorseful when in an effort to avoid responsibility… You seek to frame an innocent man and when that does not work you give false evidence that your co-accused was in fact the killer,” the chief justice said in a scathing rebuke reported by ABC News.
Palmer did not physically strike Cassius, but Justice Quinlan ruled that he was “equally responsible but not equally culpable”.
The group had also assaulted other Aboriginal teenagers in what the judge described as “so-called vigilante justice [that] was completely misdirected”.
A fourth offender, Ethan MacKenzie, was handed a two-and-a-half years jail term for his part in some of the other assaults.
In one case, a 13-year-old boy’s own crutches were used to beat him, causing bruising to his face.
Prosecutors said the group had been “hunting for kids” because somebody had damaged Brearley’s car windows.
The attack on Cassius was the culmination of a complex series of tit-for-tat events that had nothing to do with him, they said.
Justice Quinlan condemned Brearley, Palmer and Forth for their “celebration” after the assaults, calling it a “grotesque display of your complete disregard of the lives of the children you had attacked”.
In her victim impact statement on Thursday, Cassius’ mother Mechelle Turvey said the actions of the three men were racially motivated.
“Cassius was not just part of my life, he was my future,” Ms Turvey said. “There are no words that can fully capture the devastation of losing someone you love to violence.”
While Justice Quinlan did not find the attack to be motivated by race, he said the attackers’ use of racial slurs “rippled” through the Aboriginal community and created “justifiable fear”.
“The fear is real and legitimate. You are responsible for that fear,” he said.