A man who pushed a young Brit on a "dream" working holiday down a flight of stairs, causing him significant brain damage, has been sentenced to prison.

Thomas Louis Nathan, 56, pleaded guilty last month to one charge of causing grievous bodily harm with intent to injure after he pushed Josh Storer down the stairs of an Albany pub last year.

Judge Kathryn Maxwell said "one moment of aggression" had affected many lives, in sentencing Nathan today in the Auckland District Court.

Storer, then 25, lost the right half of his skull and still suffers from serious, long-term brain injuries as a result of the July 9, 2021 assault.

The two men had spoken briefly at The Albany pub on Auckland's North Shore before Nathan grabbed Storer by the front of his jacket and threw him backward with such force that he fell down a 2.6m flight of stairs, crashing head-first onto concrete below which knocked him out immediately.

Bystanders tried to give first aid and called emergency services, while Nathan and his friends left.

Nathan could not explain to police why he did it except that Storer was annoying him, according to court documents.

Thomas Louis Nathan when he pleaded guilty at the Auckland District Court in August. Photo / NZME

He told a pre-sentence report writer that Storer was "in his face" and "like a little terrier" that wouldn't let go.

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Nathan also said he wasn't aware the stairs were there, but the judge dismissed the claim saying "it defies common sense" when looking at pictures of the pub and deck where the incident happened.

"Your offending is serious," Judge Maxwell told Nathan, though there was no evidence to suggest Nathan intended to wound or cause serious bodily harm to his victim.

Nathan's family said the incident was completely out of character, Judge Maxwell said.

She pointed to letters from his family and friends describing him as a loyal man who helped and supported others, taking good care of his immediate family.

However the judge said it was an unprovoked and gratuitous attack, and this was an aggravating feature.

Being annoying was never serious enough to qualify as provocation, she said. "Never."

Storer was also vulnerable, being smaller than Nathan, intoxicated, and had his back to the stairs when he was taken by surprise and thrown, the judge said.

She sentenced Nathan to two years and five months in jail.

Storer's mother Dawn told the court she and husband Ian had never felt so helpless as parents while they waited in MIQ to see their son in hospital, after flying to his side from England.

"We would like you to take into account that this is far from over for Josh and ourselves," she said, addressing Nathan at one point as she read her victim impact statement in court.

"The extent of his injuries will be unknown for some time ... Will he be able to work again? Will he be able to come back to New Zealand? Will I have to become my son's carer?"

Doctors had to remove the right half of Storer's skull to save his life, later inserting an artificial skull panel to reconstruct his head in a second operation before he was flown home to the United Kingdom.

He is still struggling with recovery in a brain injury hospital.