Socialising with siblings, Jaman Maurice Rio believed he heard a racist slur from a group of older men outside a Brisbane casino.
When the three men in their 40s walked away after an argument, Rio and his two younger brothers followed.
The two groups got into another verbal exchange before some pushing and shoving.
Then one of the older men hit Rio's brother.
"It was at that point that you became involved physically," Judge Geraldine Dann told Rio in Brisbane District Court on Wednesday.
When one of the older men fell to the ground, Rio punched him in the head three times.
The August 2020 fight that erupted in Brisbane's CBD after 3am went for just two minutes.
However, the older men - aged 41, 46 and 49 - suffered significant injuries.
"It's important for me to outline those injuries ... are not attributable to actions by the defendant or any particular person," crown prosecutor Emily McGregor said.
"However, it is relevant to understand how badly this particular event went wrong ... they are quite serious consequences."
One man suffered a mild traumatic brain injury, with permanent damage to his sense of taste and smell.
Another was left with a fractured ankle, cut lip and a nose injury.
The 41-year-old man punched by Rio suffered an ankle complaint.
"But I am not sentencing you on the basis that you are responsible for that injury," Judge Dann added.
In his victim impact statement, the 41-year-old said he suffered anxiety and struggled to relax and go out in public, needing ongoing psychological treatment.
Defence barrister Kim Bryson said there were conflicting versions about whether the older men directed a racial slur at Rio's group during the initial argument outside the Treasury Casino.
But she said New Zealand-born Rio believed at the time they had.
Judge Dann told Rio: "To punch a man three times in the head while he is on the ground, whatever the provocation, is really inexcusable.
"You are the eldest of your ... brothers and you should have had the wisest head."
Rio breached a suspended sentence with his "gratuitously violent" offending.
Judge Dann said it was a concern that the incident occurred two months after Rio had received a suspended 18-month jail term when he pleaded guilty to grievous bodily harm.
Rio's brothers - aged 18 and 20 at the time - were sentenced for affray by a magistrate in May, the court heard.
Since the August 2020 incident Rio has been remorseful, rarely goes out and no longer drinks to excess, Ms Bryson said.
Rio is a gifted rugby league player who has played representative footy and is currently with first grade outfit Southport Tigers, the court heard.
Rio, 24, on Wednesday pleaded guilty to common assault and affray.
He was sentenced to six months' jail and ordered to pay $2000 compensation.
Judge Dann also activated the 18-month suspended prison term.
Both jail terms are to be served cumulatively, with an immediate parole release.