"You resisted then went further and punched him, then went further again and this is why you're in the dock today and going to prison... you grabbed hold of his face, then squeezed his face with your fingers,'' the judge said.
One of Whaanga's fingers entered the victim's eye, leaving him writhing on the floor in pain.
Surgeons had to remove the victim's retina to repair the damage to his eye, but Crown prosecutor Richard Smith said he had since made a full recovery.
The victim required three weeks off work to recuperate and the prescription for his glasses had changed as a result of the trauma.
Once informed about Whaanga's mental health difficulties, the man was compassionate but wrote in a statement he considered the attack "particularly savage''.
The court heard it was not the first time the defendant had been in court for his violent outbursts.
Judge Phillips took issue with Probation's assessment of Whaanga as a medium risk of future offending, noting it was his fourth such conviction in five years.
The man explained the original flashpoint had occurred because someone had thrown a drink at him.
Whaanga claimed he had "lashed out'' and had not intentionally gouged the victim's eye.
That was accepted by Smith, who also acknowledged the defendant's psychological problems would make a term of imprisonment harder for him than others.
Whaanga's parents were in court to support him but said they would not forward their home for an electronically monitored sentence.
Judge Phillips too highlighted the defendant's issues, but said they would always be exacerbated if he resorted to drug and alcohol use.
Whaanga was ordered to pay the victim $1800.
That would be funded by his parents, the court heard.
