A 13-year-old East Coast boy has become one of New Zealand's youngest killers after admitting a charge of manslaughter today.

The teenager was 12 when he shot an 11-year-old boy dead last year. He today pleaded guilty to manslaughter at the High Court at Gisborne, on what was scheduled to be the first day of his murder trial.

A deal between his lawyers and the Crown for him to plead guilty to the lesser charge was reached several weeks ago.

The 13-year-old's father was in the dock with his son today and pleaded guilty to attempting to pervert the course of justice. He was originally charged with the more serious charge of being an accessory to murder. Both have name suppression, at least until they are sentenced on December 7.

The father's charge relates to his actions in the aftermath of the shooting. Both father and son have been bailed until their sentencing hearing.

Emotional family members of the victim filled the court this morning to hear the guilty pleas.

The shooting occurred in July last year, at a remote farm inland from Tolaga Bay, but the father and son were only arrested earlier this year.

The two boys were arguing about who should take a knife on a hunting trip.

The bullet entered the victim's left shoulder and went through his right armpit, said Crown prosecutor Russell Collins. His trachea was destroyed and he died immediately.

The victim has name suppression.

The boy was originally charged with murder, but Justice Forrest Miller said an investigation by a police armourer could not exclude the possibility that the firearm was discharged accidentally.

Justice Miller said the son was still a child and could only be convicted of a crime if he was capable of telling the difference between right and wrong.

Evidence from a doctor and four teachers was that he was capable of making that distinction.

Crown prosecutor Russell Collins said the defendant and the victim were staying in a small cottage at a farm where his father worked.

The defendant, the victim and two other youths had been hunting on the farm with the 7.62mm semi-automatic.

The defendant was familiar with the weapon, said Mr Collins.

At 4.15pm the defendant, the victim and another youth were in the lounge of a cottage. The third person was playing on a PlayStation while the defendant sat behind the victim.

The firearm was under the table when the youths began discussing whether to go hunting again.

An argument began.

The victim called the defendant an arsehole.

"I'll shoot you," said the defendant.

The victim laughed and the defendant took the rifle from the table and loaded it with two bullets.

The victim told the defendant to get the gun out of the way.

The defendant swung the gun around towards the victim and told him he would no longer get cheeky.

The victim turned away and the firearm went off.

The father told his son and the other witness to tell police that the victim had the firearm and that it accidentally discharged as the accused tried to remove it from him, Mr Collins said.

The father and son were remanded for sentencing on December 7.

Justice Miller ordered a pre-sentence report and home detention/community detention appendices but said that was no indication of the final sentence.

Strict suppression orders have been placed on the case because of the boy's age.

NZ's youngest killers over the past 20 years

NZ's youngest killers over the last 20 years

* July 17, 2009: A 12-year-old East Coast boy shoots his 11-year-old friend with a 7.62mm semi-automatic firearm after the two boys argued about who should take a knife on a hunting trip. He later admits manslaughter and will be sentenced in December.

* January 26, 2008: Kalem Ames, 14, stabbed Tokoroa man Shayne Pita Walker, 22, to death in the town. He was charged with murder, and a jury found him guilty of manslaughter.

* January 16, 2008: Jahche Broughton, 15, beat Scottish tourist Karen Aim, 27 to death with a baseball bat, just 12 days after bashing apprentice chef Zara Schofield, 19, on the head with a rock . At the age of 15, he was sentenced to life imprisonment for Ms Aim's murder, with a minimum non-parole term of 12-1/2 years, and six years' imprisonment for wounding Ms Schofield, to be served concurrently.

* October 2007: Petani Fa'avae, 16, stabbed to death Manaola Kaumeafaiva, 14, outside Avondale College during a church dance. He was found guilty of murder and jailed for 11 years.

* March 2007: Rua Edwardson, 16, was found guilty of the manslaughter of her former friend, Melissa Puhi.

* January 13, 2007: Kori Trevithick, 15, stabbed Auckland widow Doreen Reed, 77, over 21 times as she slept in her bed at Birkdale. He was convicted of murder.

* July 2006: Ngatai Rewiti, 14, was convicted of the manslaughter of Christopher Currie after he dropped a concrete slab from an Auckland motorway overbridge on to Mr Currie's car.

* May 2004: James Hamilton and Hori Slade, both 17, were convicted of murdering Whangerei forestry worker Richard Harcombe.

* February 2003: Renee Kara O'Brien was convicted of the murder Waitara truck driver Kenneth Pigott. O'Brien was 14. Her co-offenders - Puti Irene Health Maxwell and Kararina Makere Te Tauna, both 14 - were convicted of manslaughter.

* August 2003: John Michael Jaymain Wharekura, 17, was found guilty of stabbing to death Tanya Burr at her Rotorua flat.

* September 2002: Daniel Luff, 17, was convicted of killing Detective Constable Duncan Taylor near Palmerston North. He also pleaded guilty to the attempted murder of Detective Jeanette Park.

* September 2001: Bailey Junior Kurariki was 12 when he took part in the fatal beating of pizza deliverer Michael Choy, 40. He was charged with murder and convicted of manslaughter. In August 2002, Alexander Peihopa, 16, and Whatarangi Rawiri, 17, were found guilty of murdering Mr Choy.

* May 1991: A 13-year-old Wellington schoolboy was convicted of the murder of nurse Rachel Bennett, who was stabbed 16 times.