A man who fatally headbutted a stranger during a dispute over a supermarket car park claimed to police the victim had simply walked into his head.

Emilio Richard Mac Tanirau Whaanga today pleaded guilty to manslaughter in the High Court at Wellington over the December 3 incident.

The 55-year-old victim, whose name remains suppressed, had taken the day off work to drive his elderly mother to several appointments.

Meanwhile, Tanirau Whaanga, 23, had been trying to buy nappies for his 1-year-old child and had earlier visited another supermarket to find they were out of stock.

Both parties drove into the Countdown car park on Knights Rd in Lower Hutt about 11.30am, the summary of facts said.

The victim had his mother's wheelchair in the boot and had her mobility parking permit in the front windscreen. He had intended to park in a disability space, but these were occupied. He instead parked in a park designated for parents.

There are no by-laws in Lower Hutt preventing a person from parking in a parents' park.

The victim left his mother in the car so he could go inside and buy lunch for them, and to get her medication from the pharmacy inside the Queensgate mall. She was unable to leave the car without help.

Tanirau Whaanga stopped near the victim's car for some time before moving on and parking in a nearby vacant spot.

He then walked up to the victim, yelling out to him and pointing at the victim's car.

"As seen on CCTV, the victim began using hand gestures that indicated he was explaining something," the summary of facts said.

"The victim pointed towards the disabled carparks and to his vehicle in an attempt to explain his reason for parking in the parents' carpark with a mobility permit, and the fact that his mother was in the car with him."

Tanirau Whaanga closed the space between them, getting "within the victim's personal space" and causing him to step back. They spoke for a few seconds more.

"The defendant, still in the victim's personal space, stepped back with his left foot, and changed his weight on to his right foot.

"He extended his arms out from his body and thrust his head forward, headbutting the victim, connecting fully with the victim's head."

The victim was instantly knocked out and fell to the ground.

Tanirau Whaanga immediately turned away with his arms still extended out and walked into the store, taking large strides. He briefly turned to look at where the victim lay unconscious before continuing to follow his partner and child into the supermarket.

Members of the public rushed to the victim's aid, but he remained unresponsive and suffered about 15 minutes of seizures.

About two minutes after the headbutt, Tanirau Whaanga came back out of the supermarket.

"[He] walked past the victim and members of the public without stopping or giving any attention to the scene," the summary said.

Tanirau Whaanga got into his car and started to reverse out of the park until he realised he was blocked in by a taxi. He got out and asked the taxi driver to move his car, at which point a member of the public intervened and told Tanirau Whaanga to stay where he was, as police were on the way.

Tanirau Whaanga agreed to stay where he was, and was arrested when police arrived at the scene.

The victim was taken to hospital with multiple skull fractures, and was taken off life support the follow day. He died due to the injuries caused by Whaanga.

According to the summary, Tanirau Whaanga told police he had "flinched", and the victim had walked into his head. He also said he thought the victim was going to attack him.

"The defendant expressed regret for the incident, questioning that it would impact on his future employment prospects," the summary said.

At the time of the attack he was on a one-year intensive supervision sentence for injuring another person with intent.

Tanirau Whaanga will be sentenced in April.