Stephen Francis Radnoty died in Dunedin Hospital that night.
Matthew Bryce Larson (23), employed, pleaded guilty in the Dunedin District Court yesterday to a charge of manslaughter after the incident on March 12.
He was convicted, given a "three strikes" warning and remanded in custody by Judge Rob Murfitt, of New Plymouth, for sentence in the High Court at Dunedin on July 6.
Mrs Radnoty said she was pleased Larson had pleaded guilty, because it meant family members would be spared the ordeal of the matter going to trial.
"It would have dragged everything out for such a long time." She would attend Larson's sentencing and intended to make a victim impact statement.
Prosecutor Sergeant Graeme Evans told the court Mr Radnoty died as a result of a punch to his head while waiting for food at the George St restaurant about 4am.
Mr Radnoty had been celebrating with friends at an inner-city tavern, as he and his partner were preparing to leave Dunedin to start a new life in Roxburgh.
The celebrations over, the victim, his partner and a friend drove to McDonald's. Mr Radnoty's partner waited in the car.
As Mr Radnoty placed his order, his friend became involved in an argument with an associate of Larson, Sgt Evans said. McDonald's security staff were able to calm the situation.
After placing his order, Mr Radnoty waited beside the back wall, and watched his friend shake hands with a person involved in the argument.
"[Mr Radnoty] was still standing against the back wall, waiting for his order, when, without any provocation or warning, the accused punched him on the right side of the head," Sgt Evans said.
"The punch thrown by the accused was delivered with such force that it caused the victim to fall backwards on to the floor. As he landed, the back of the victim's head crashed heavily on to the tiled floor."
Mr Radnoty, who did not know Larson, was knocked unconscious.
Larson was restrained by security staff, but broke free and attempted to leave.
Mr Radnoty's friend and McDonald's staff prevented him from leaving, holding him until police arrived.
"The blow to the back of the victim's head from the impact on the tiled floor, after being punched by the accused, was so severe that it resulted in a life-threatening haemorrhage to the brain."
The victim was resuscitated by paramedics inside the restaurant and taken to Dunedin Hospital. He never regained consciousness and died later that night.
Larson admitted punching the victim.
"The defendant stated that he could have walked away but he didn't, and all he could hear was what the victim had said - that's why he turned around and punched him," Sgt Evans said.
A McDonald's spokesman said a security investigation launched after Mr Radnoty's death was nearly completed.
The findings would not be released publicly, but would be discussed with Mrs Radnoty.
