A man who died after he was punched to the ground during an argument during Rugby World Cup celebrations at Auckland's Viaduct Harbour was out cold before he even hit the ground, a witness has told a court.

Billy Dawson, a Sky TV employee, had been celebrating the end of the Rugby World Cup pool play with workmates when he was involved in an altercation outside the Spy Bar in October last year.

The Crown says the man who threw the punch was Kit John Murray. His manslaughter trial began in the High Court at Auckland today.

His lawyer Andrew Speed says his client only pushed Mr Dawson and has asked the jury to consider self-defence.

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DJ Jeremy Kingi told the court he saw Mr Dawson being slapped in the face on the ground outside the club by another man.

Mr Kingi said he knew one of the men involved and was trying to get him in a taxi when Mr Dawson shoved Murray.

He said Murray responded by shoving Mr Dawson back and then punched him in the face.

"It was pretty clear that it knocked him out. He was knocked out after he received the punch and then he went down."

Mr Dawson had a broken jaw and his skull was fractured by the impact with the pavement.

He died the following day at Auckland Hospital.

Under cross-examination from Mr Speed, Mr Kingi said that he heard Murray saying "just walk on, move on" before he struck out.

He described Mr Dawson as "being prepared to start a fight".

The court also heard from Marcus Hildyard - an associate producer at Sky - who described his workmate as being in a "happy sort of mood" earlier in the night.

He said the day had started with a BBQ, beers and a bit of backyard cricket. The friends had a few drinks before hitting town.

He said the friends headed to Danny Doolan's Irish bar at the Viaduct.

"Billy had a chat with a few of the ladies but he had no luck with them."

Mr Hildyard said Mr Dawson had gone into Spy Bar and they lost contact with him.

Earlier, Crown prosecutor Natalie Walker said Mr Dawson had bumped in to a man as he left Spy Bar.

She said Mr Dawson had made a racist remark before his legs were swept from under him and he was held down and slapped in the face.

Another man put his boot in Mr Dawson's face before bar security stepped in and broke it up.

She said Mr Dawson was walking away when Murray approached him and without provocation punched him in the face.

Mr Dawson fell straight back without being able to break his fall

She said witnesses described the sound of his head hitting the pavement as a "sickening thud".

Ms Walker said that as witnesses went to help Mr Dawson Murray and his friends got into a taxi to continue drinking at other bars.

Police caught up with Murray and his friends in the early hours of the next day.

The jurors will hear evidence from 52 witnesses over the next two weeks.