Two Whanganui brothers accused of knocking a relative semi-conscious and rolling his body down a bank toward the Whanganui River have been discharged on a count of attempted murder.

The charge faced by Michael Wesley Kumeroa, 27, and Kelvin Martin Kumeroa, 30, was replaced in the High Court in Wellington by one of injuring their 22-year-old second cousin Mark Bartlett with intent to cause grievous bodily harm.

The brothers also had a charge of injuring another relative, Javan Kapene Te Amo, with intent to cause grievous bodily harm replaced with one of injuring him with intent to injure.

Michael Kumeroa changed his pleas to guilty on the substituted counts.

When the trial opened at the start of last week, he had admitted a charge of common assault on Fiatau Faalite, a stranger whom he punched in the face at a Whanganui service station some hours before the other offences.

The charges arose from a night of drinking and spotting cannabis at the beginning of December 2009.

Both men were accused of beating up Mr Bartlett, then taking the semi-conscious man by car to the edge of the Whanganui River and rolling him down a bank.

The court heard that Michael Kumeroa suddenly "uppercutted" Mr Te Amo in the face with his fist when Mr Te Amo refused to use cannabis.

Mr Bartlett said in evidence that he was trying to help the injured man when the brothers attacked him, punching and stomping him in the head and body.

He remembered later being dragged out of a car in the pitch dark, pulled along by his legs and bouncing face-down through gravel for several metres.

Then, he said, he was pushed over a cliff and recalled rolling down the river bank.

Mr Bartlett told of awaking several hours later, injured and sore, with his legs immersed in the cold river. He crawled up the bank and managed to drag himself along the road to a house where the owners called the police and an ambulance.

Summing up the case against Kelvin Kumeroa to the jury , Crown prosecutor Lance Rowe referred to the troubled past of the brothers, who had served jail terms for killing a man in Whanganui during a June 2000 street fight.

Paul Amer, a 41-year-old father of six, was pushed to the ground and kicked repeatedly in the head, and later died of his injuries.

In a plea bargain in early 2001, the brothers avoided a murder trial by pleading guilty to manslaughter.

Mr Rowe said Kelvin Kumeroa had a history of attacking people, and knew what could happen. He said the pair also had other convictions for violent behaviour.

The jury retired to consider its verdicts mid-afternoon.