The defendant was on remand at Christchurch Men's Prison in June 2017 when he attacked his cell mate while the man ate his lunch.
Bartlett punched and kicked the victim while he was on the floor and the violence only ceased when the man was able to reach a panic alarm to alert guards.
A move to Otago Corrections Facility did not quell his violent tendencies.
On April 1 last year, he, with Hawi Kingi (22) and Jahmyn Inia (25), entered the exercise yard making gang gestures.
Bartlett set upon another inmate, who was stretching at the time.
Once the victim was on the floor the blows continued and even when Corrections officers had intervened, Bartlett walked over and landed another blow.
Justice Nation called it "cowardly".
"The whole attack was vicious and unprovoked, but what marks out your offending was the way, after applause from other prisoners, you walked over and again stomped on the victim's head," he said.
The trio were all charged and transported to court for their first appearance on June 8 and while they waited in a Dunedin courthouse cell, Bartlett laid into Kingi.
Again, he stomped on the man's head until authorities came to his aid.
Bartlett later explained: "When people stuff up there are big consequences."
He referred to the cell violence as "a stupid test", orchestrated by the gang with whom he was affiliated.
Bartlett was adamant when speaking to psychologists that he would not kill for the gang but the judge said when targeting someone's head the risk was great.
The reports on the defendant outlined several issues that had shaped his life.
Bartlett spoke about the love he had for his late mother, who had attempted to shield him from an abusive upbringing.
"During her life, she made decisions and took action which must have been brave but which came with considerable consequences for her," Justice Nation said. "She had the strength of character to do that. You need to show that you have the same strength of character."
The court heard Bartlett's hearing difficulties had also led to antisocial behaviour.
Despite that, his mother had taught him te reo as a child and he had a strong attachments to his Maori culture through youth groups and kapa haka.
Bartlett was keen to return to Hawkes Bay to re-engage with his roots.
Justice Nation sentenced him to seven years' imprisonment, taking into account theft charges and breaches of sentence the defendant also faced.
While he refused to impose preventive detention, he warned Bartlett how precarious his position was.
"If you are involved in further incidents of violence in the prison, especially if they involve attacks to the head and neck area, the likelihood of preventive detention will be much stronger."
Kingi was earlier jailed for two and a-half years; Inia for two years, over the exercise-yard incident.
