In the dock today, Oka-Murray kept his head bowed as the gruesome details of offending were read to the court.
The trouble started when a Levin bar was cleared because of brawling patrons.
Mr Diffey, who was not involved in the trouble, and Oka-Murray were among the people who found themselves on the footpath.
The initial scuffle was between Oka-Murray's cousin and the brother of one of Mr Diffey's mates.
But Oka-Murray somehow entered the fray and punched Mr Diffey in the head from behind. As he fell to the ground, Mr Diffey's skull smashed into the footpath.
The lack of remorse, denials of any involvement will never allow us to forgive the defendant.
He suffered severe brain trauma and was flown to Wellington Hospital for immediate surgery. Mr Diffey remained in a critical condition and his family was even told his chances of survival were slim.
He had surgery a second time and, after a week in intensive care, was admitted to a rehab centre. Since he left he still undergoes occupational therapy and, in a victim's statement to the court today, he described his ongoing mental and physical pain he suffered after the punch.
He said it was "not in my nature to fight".
"I have never been in a fight before and I have never thrown a punch. I don't understand why he felt the need to deliver a near fatal hit," he said.
Mr Diffey said his injury changed his life forever.
Before the incident, Mr Diffey was a promising carpenter who worked for his father's business. He dreamed of building houses but now knows that will never come true because his injury means he can't work full-time.
Instead, he had a part-time cleaning job for his dad.
"I'm sweeping floors and tidying building sites a few hours a week. This mundane work is mentally exhausting," he said in front of his family who were in the public gallery supporting him.
An emotional Mr Diffey couldn't finish reading his statement and a family member took over.
His mother, Tania Diffey, also read a victim's statement, recalling how "helpless" she felt as her son was fighting for his life in hospital.
After waking up from surgery, Mrs Diffey said her son found speaking and eating difficult and he was confused by his surroundings.
"He would wonder the ward looking for his bedroom."
She said staff locked the ward to keep him safe and other patients was told of his confused state of mind. He also had a tough time at the rehab centre.
"He hated it there and we hated leaving him there. He would beg us to take him home," she said.
"After he could tell the difference between boiling water and cold water and how to cross the road they let us take him home."
Through "hard work and tenacious attitude", her son's condition had improved.
But she said the family wouldn't forgive Oka-Murray.
"The lack of remorse, denials of any involvement will never allow us to forgive the defendant."
Defence lawyer Mr Robert Lithgow said Oka-Murray, now 20, was simply a "bystander watching his cousin being beaten on the ground from the brother of the victim's friend".
Oka-Murray had immediately confessed to punching someone but at first didn't think it was Mr Diffey.
Judge Chris Tuohy said serious brain injuries could have permanent effects.
He accepted Oka-Murray was "very remorseful" - as shown when he went to the police station voluntarily to make a statement.
When sentencing Oka-Murray, the judge took into account the 20-year-old's full-time job, age and lack of other convictions.
As the home detention sentence was delivered, Oka-Murray's family sobbed in the back of the court.
