UPDATE: A Te Puke man who carried out a fatal prison-ordered bashing over a gang drug debt should never have been found guilty of murder, his lawyer told the Court of Appeal today.

Kickboxer Witeri Ahomiro Neketai, 37, is serving at least 13 years jail for the murder of 44-year-old Tauranga businessman Gary Kimura outside his Bellevue home in 2011.

Witeri Ahomiro Neketai (in white) during sentencing in the High Court at Rotorua.

The attack was carried out on the order of Auckland man Joseph Rewiri over a $35,000 drug debt owed to a prisoner Rewiri was serving with at the time.

Neketai's one punch caused the 109kg Kimura to fall and hit his head on the concrete. He then kicked him in the face while on the ground.

In the Court of Appeal in Wellington today, Neketai's lawyer Christopher Tennet argued it was unusual to have a murder conviction for a one-punch killing.

He argued a manslaughter finding would have been the correct finding, as Neketai could not have known his punch would be fatal.

Kimura was knocked to the ground outside his Margaret Road home on October 5, 2011 and taken to hospital, where he died six weeks later on his birthday of November 16.

He died from a skull fracture, but there was no evidence that came from other blows other than hitting his head on the concrete, the court heard.

Neketai was found guilty of murder by a jury trial in July 2012 and subsequently sentenced to life in prison with a non-parole period of 13 years.

Tennet said the jury had not been properly directed on how to regard the subsequent kick.

Neketai had even put Kimura in the recovery position before leaving, Mr Tennet said.

"To some extent he is then victim of his own fitness and bulk."

Crown lawyer Peter Marshall told the court the jury had seen enough evidence to make up its mind about murderous intent or Neketai's recklessness about the attack.

The justices reserved their decision.