An accidental step was all it took for a teenager to allegedly attack a Hawke's Bay father and inflict the critical injuries that led to his death, a court has heard.

Johnnie Puna, 19, is standing trial in the High Court at Napier this week before Justice Helen Cull after pleading not guilty to the murder of Haumoana man Mark Geoffrey Beale.

The 45-year-old was found unconscious with critical injuries near his home in the seaside settlement about 7am on Waitangi Day and died in Hawke's Bay Hospital the next morning.

Puna was charged with murder on February 24.

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Crown prosecutor Steve Manning opened the trial yesterday morning by telling the jury of eight men and four women that the trial would pivot on Puna's intentions.

"What would otherwise be manslaughter becomes murder if the person delivering the blows, in this case Mr Puna, does it with murderous intent," Mr Manning said.

The court heard the defendant met the father of two just hours before he meted out a series of fatal blows to Mr Beale's head.

Puna, his uncle and cousin had been socialising and drinking pre-mixed alcohol at the Tukituki river mouth that February evening, the prosecutor said.

Mr Beale, who lived on Grange Rd several hundred metres from where he was later found, was a loner and had a long-standing problem with alcoholism, the jury heard.

His eldest son, Jared Beale, corroborated this lifestyle.

He told the court his father had an "intimate" relationship with alcohol, vodka being his drink of choice, and socialised when fishing in the area.

"I think his circle of friends in the area was low so I believe he'd go to the beach or those fishing spots where people would be line fishing and that would be his social activity, his way of interacting with people."

Mr Beale had been drinking that February evening and met the trio, the defendant included, at the river mouth, Mr Manning said.

At one point Puna accompanied Mr Beale to his home to get more alcohol and, upon their return Mr Beale tripped and instinctively reached his hands out; pulling Puna down with him, the prosecutor said.

"It seems Mr Puna chose to attack Mr Beale for an innocuous and simple reason. Mr Beale accidentally fell over, pulling Mr Puna to the ground and causing him to receive a minor cut to his lip."

This accidental trip, Mr Manning said, was the "catalyst" to what led to Mr Beale's death.

In a state of anger Puna delivered a series of full forced kicks to Mr Beale's face and head until the man was knocked out, before waiting for him to regain consciousness to further assault him, the court heard.

Mr Manning said the attack was not carried out in a frenzied state but over time, with gaps between the blows.

The jury heard Mr Beale was in a bloodied state making "gurgling noises" when the trio left the area.

Rarotonga national Mati Atiau testified that he saw a Pakeha man lying on the ground with bruising around his face.

Through the aid of a translator, he told the court about asking one of the three men he saw near the man if he was okay.

"One of them said, 'leave him alone, he's all right. He's too drunk'."

The 24-year-old was among those who found Mr Beale the next morning and said he was close to tears when he helped carry him out of the area.

Puna's defence lawyer, Eric Forster, told the jury his client did not dispute that he caused the injuries to Mr Beale, but said they were not inflicted with murderous intent.

The defence would argue Puna consumed too much of the home brew vodka Mr Beale had shared with him and then shoved him before the violence ensued.

The trial is expected to last one week, with the Crown calling 16 witnesses.