A teenager jailed for his part in a brutal attack that left an off-duty police officer concussed with a broken jaw is appealing his sentence.

Blake Hollins-Apiata shook hands with a mate after the attack, who congratulated him and said "you just hit a cop".

Hollins-Apiata assaulted police officer Tyson Young at a birthday party in July last year, along with a group of friends who were dealt with in the youth court.

He was sentenced earlier this year to two years and six months in prison for wounding with intent to cause grievous bodily harm, which his lawyer Julian Hannam today said was too high.

The assault happened in Waitara, north Taranaki at an 18th birthday party for one of the officer's relatives.

Hollins-Apiata and his friends were also guests at the party, which started wrapping up about 11.30pm.

The then-17-year-old and his friends approached a vehicle and encouraged the driver to perform a skid.

Young overheard this and discouraged the group, warning them he was a police officer and other officers lived nearby.

The group told Young they didn't "give a f***" that he was an officer.

Hollins-Apiata then punched Young in the jaw, knocking him to the ground, the summary of facts said.

He and the group then continued to assault him as he lay injured, only stopping when Young's friend intervened and dragged him away.

Moments later, Hollins-Apiata was overheard speaking to his friend.

"[They] congratulated one another saying 'you just hit a cop' and proudly shook hands with each other," the summary said.

Young, 26, needed surgery for his jaw and a follow up surgery due to complications. He now suffers numbing to his face and tongue, and slurred speech at times.

He also suffers memory loss, severe headaches, vertigo, lack of energy, loss of balance, and slowed reactions.

He returned to work in March this year but on a gradual basis for only three hours, three days a week.

Hollins-Apiata appealed his sentenced in the High Court at Wellington today, with his lawyer arguing the starting point for the sentence was too high.

He said the sentencing judge should have adopted a starting point of 3.5 years and given larger discounts for the guilty plea and his youth.

Hannam argued his client should not have received a prison sentence, which had "thrown [him] into the arms of serious criminal offenders while in custody".

The court should also take into account other factors in Hollins-Apiata's life such as his "seriously recidivist biological father".

Crown lawyer Justin Marinovich said the sentencing judge had actually taken a slightly lighter approach to sentencing than he could have, when factoring in the multiple attackers, imbalance of power and attacks to the victim's head.

He rejected Hannam's argument about Hollins-Apiata's father, saying he had a "pro-social stepfather" and "good influences" in his life.

"He's had very little contact with his father . . . that void's been filled with someone who's certainly tried to keep the appellant on track."

Marinovich cautioned against "credit creep" where judges were encouraged to continue adding higher discounts to a sentence in order to push it below a threshold where it could then be substituted for home detention.

In response to the comments about the "pro-social stepfather", Hannam said he had acted as a defence lawyer for Hollins-Apiata's stepfather in the past and he was "not exactly an angelic influence".

He said his mother had issues with drug use, but the couple had pulled themselves together to provide a stable home life for Hollins-Apiata.

Justice Helen McQueen reserved her decision.