Thomas Nathan could not explain why he threw a man half his age down the stairs with such force it spilt his head open - besides the fact the young Englishman was annoying him at the pub that night.
Joshua Storer lost the right half of his skull and still suffers from serious, long-term brain injuries as a result of the assault in 2021.
Court documents released for the first time today reveal how Storer was slight and "considerably shorter" than 56-year-old Nathan, who was much larger and 190cm tall.
Nathan pleaded guilty to one charge of causing grievous bodily harm with intent to injure at the Auckland District Court this morning before Judge June Jelas.
He came with his wife, who curled her arm around his while they waited in court for his hearing.
Nathan and Storer, who was then 25, did not know each other but spoke briefly several times while drinking at Auckland's Albany Pub on the night of July 9, 2021.
Just after 11pm, the two were standing and talking on the deck area of the pub next to a flight of eight stairs.
At one point, Nathan reached out and grabbed Storer by the front of his jacket. In one movement, he turned the younger man slightly and threw him backwards down the stairs.
Storer was thrown with such force that he fell backwards and head-first down the entire flight of stairs, a distance of some 2.6 metres.
He landed on the back of his head and shoulders on the concrete pad at the bottom of the stairs, his feet pointing upwards.
He was knocked out immediately, with a 15cm laceration on the back of his head that started to bleed.
Bystanders tried to give first aid and call emergency services, while Nathan and his friends left.
When asked, Nathan admitted he threw Storer down the stairs and demonstrated the movement he used, but was unable to give any explanation other than that the young Englishman had been annoying to him.
Storer suffered extremely serious injuries including a traumatic brain bleed and skull fractures.
Doctors had to remove the right half of his skull to save his life, later inserting an artificial skull panel to reconstruct his head in a second operation.
He was in a coma for a month in ICU, and spent another six weeks in rehab before flying home to the UK to continue his recovery last December.
Storer's parents Dawn and Ian say they are pleased Nathan has finally entered a plea more than a year after the incident.
"We just really hope the sentence reflects the severity of the life-changing injury that Josh has sustained," they told Open Justice.
Storer has been in a secure brain injury hospital since February and they believe he could be there for some time.
He was keen to return to his life in New Zealand, but his recovery was slow and the whole family was struggling. "He's desperate to come back," Ian said earlier this year.
The injuries damaged the young man's frontal lobe and his ability to manage his emotions, reasoning and impulses.
"If something went against him he could be very impulsive and very angry straight away, like a child," his father said.
Nathan faces a maximum penalty of seven years in jail and was given a first-strike warning after he pleaded guilty in court this morning. He was granted bail with a night-time curfew until his sentencing in September.
He and his wife held hands as they left the courtroom.