When 50-year-old Kevin Patrick Hay - a large-statured man with mixed-martial arts training and a history of mental health and drug issues - showed up uninvited at a North Shore home minutes before he was stabbed to death in 2022, CCTV clearly shows he was the initial aggressor who started throwing punches, jurors were told today.

But Hay had already lost the upper hand in the fight, with 58-year-old Darren Troy Couper on top of him and in control, when Peter Robert Greene intervened with a knife and fatally stabbed Hay through the liver, prosecutors said today as the pair’s High Court at Auckland trial began.

Green, 57, is charged with murder while co-defendant Couper is facing a charge of wounding with intent to cause grievous bodily harm for allegedly continuing to inflict blows after Hay was incapacitated. Both men have pleaded not guilty, claiming self-defence.

“There seemed to have been some bad blood,” Crown prosecutor Belle Archibald told jurors during her opening address. “For whatever reason it is clear they did not like one another.”

Neither defendant would have been helpless against Hay’s fight training, she suggested, pointing out that Greene and Couper had both trained at a martial arts club and “dabbled in kung fu”.

Authorities allege Hay walked up to the Oceanview Rd home, near the border between Northcote and Hillcrest, and started pounding on the garage door around 9pm on August 16, 2022. The co-defendants were inside the home with a third man smoking methamphetamine together, Archibald told jurors.

Police at the scene of Kevin Hay's death on Ocean View Rd, Hillcrest, in August 2022. Photo / Ben Leahy

It’s not in dispute that Couper’s initial decision to fight back was both defensive and legal, she conceded before adding: “But a person’s motives can change ... and a person can start using unreasonable force.”

When Hay stopped throwing punches, that level of force was no longer reasonable, she said, explaining that Couper ended up throwing roughly 20 more punches over the next four minutes, as well as kicking Hay in the head and stomping on his hand.

While the two were grappling on the ground, Green arrived with two knives - one of which he used to stab Hay about two minutes into the fight, prosecutors allege.

Hay was eventually able to get up and leave the address, but he collapsed on a grass berm a short distance away. His partner found him dead about 30 minutes later.

When interviewed by police a week later, Greene said he told police he stabbed because “in effect, he wanted to stop Mr Hay from coming around the address”, Archibald said, suggesting to jurors that such an explanation is not in line with self-defence.

Kevin Hay died on August 16, 2022, after an incident at an Ocean View Rd home on Auckland's North Shore.

Defence lawyer Julie-Anne Kincade, who represents Greene, said jurors will have to put themselves in her client’s shoes amid the attack from an uninvited guest. They will also have to consider past interactions, including previous threats and an incident a month earlier in which Hay showed up at the same address with a baseball bat and smashed a car window.

Lawyer Marie Dyhrberg KC, who represents Couper, also emphasised during her brief opening statement that jurors will have to consider her client’s state of mind.

“He had no choice... He fought that night for his life from start to finish,” she said, describing the confrontation as “the fight of his life with a man who he knew ... had serious mental health problems [and] was capable of really hurting people through martial arts”.

“In his mind, Mr Hay meant trouble. He was there for trouble.”

Dyhrberg said her client should also be acquitted of two other charges he faces: the alleged aggravated assaults of two police officers who showed up at his home later that night trying to serve a search warrant. One officer suffered light bruising on his face after the door was slammed in his face, while another officer was punched by Couper, prosecutors said.

But Couper hadn’t yet learned that Hay had died when police showed up with assault rifles, tasers, pepper spray and a police dog, Dyhrberg said.

“Mr Couper honestly believed in all the circumstances ... he was justified in what he did because police were way out of line in what they did that night,” she said. “They were not acting reasonably. They were not acting in accordance with the law.

“It was excessive force. It was unreasonable and it was unlawful.”

The trial is set to continue tomorrow before Justice Peter Andrew and the jury.

Craig Kapitan is an Auckland-based journalist covering courts and justice. He joined the Herald in 2021 and has reported on courts since 2002 in three newsrooms in the US and New Zealand.