Wirihana accepts he caused the stab wound, but told the police it was an unintentional act of self-defence while he was trying to remove the keys from Mason's vehicle after it was driven at him at speed and injured his partner, the jury heard.
During her opening address, Crown prosecutor Ella Collis told the jury the defendant was drinking and socialising at his partner's address on the day of the alleged murder, along with several other family members.
She said Wirihana offended a member of the group who left the address and went to a nearby bus stop. The woman called the police several times about Wirihana's behaviour and she also told Mason and three other men she knew when they drove past.
The woman got into Mason's vehicle and once back at the address, Heke and Mason's son confronted the defendant, Collis said.
A fight broke out and punches were exchanged. During the altercation, Wirihana got Heke down on the ground and lay on top of him, she alleged.
Wirihana bit down on Heke's left ear, ripping off a large portion, then bit down on his nose causing "a portion of flesh to come loose".
"The defendant then entered the house and retrieved two knives from the kitchen and began chasing the two men he was fighting with around the property."
Collis said "in the midst of the chaotic scene" Mason's vehicle was driven towards Wirihana then reversed, pinning Wirihana's partner's leg against another vehicle. Mason was in the passenger seat.
An angry Wirihana then entered the vehicle from the passenger's side and inflicted a "single and ultimately fatal" stab wound to Mason's neck with a knife, Collis said.
Mason was treated at Whakatane Hospital before being flown to Waikato Hospital and operated on, but he died at 8.27am on December 26.
Collis told the jury a forensic pathologist who conducted the autopsy found the direct cause of Mason's death was a single 6cm-deep stab wound to his neck.
Collis said Wirihana's statement to the police that he had accidentally slashed Mason's neck while reaching for his car keys raised the issue of self-defence.
"However, no matter how sorry or regretful the defendant was after the act, just saying he lost it in the heat of the moment is not sufficient to negate murderous intent.
"Either when the defendant stabbed Mr Mason he actually intended to cause his death or intended to cause him a bodily injury which he knew was likely to kill him and diced with the risk anyway regardless of the consequences."
The jury also heard Wirihana was treated for injuries to his face, chest and head and Heke was treated for facial injuries.
Adam Holland, one of Wirihana's lawyers, urged the jury to consider carefully the context of the confrontation and the threat his client believed he was under when he bit Heke.
Holland said from the defence perspective, Wirihana was "essentially set upon" by a carload of men whose motives he did not know.
The jury also needed to consider what sort of injuries Wirihana suffered, and whether he had tried to flee the scene, he said.
"In respect to the grievous bodily harm charge, context is everything because Mr Wirihana maintains that his actions in biting Mr Heke were essentially a desperate act of self-defence."
Holland said the jury must acquit Wirihana of that charge if they believed his response was reasonable in the circumstances.
He said in terms of the murder charge there was again a "self-defence element" as the stabbing happened in the aftermath of the incident involving the vehicle that left his partner injured.
"At no time do we say did Mr Wirihana ever form an actual intention to murder the deceased ... it was an unintentional single stab wound while he was struggling with Mr Mason as he desperately tried to disable his vehicle by grabbing the car keys."
The trial, presided over by Justice Rebecca Edwards, is expected to take two weeks.
