Jara Lee Rice Dix, 22, and Benjamin Keith Judge, 29, could spend up to three years and four months in prison for kidnapping and intentionally causing injury on March 15 last year.

They were part of a plan to kidnap a 19-year-old man, assault him and leave him tied to a tree in the bush for days over his perceived mistreatment of Dix's niece.

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Dix and Judge took the victim into bushland at One Tree Hill on the night of March 15 on the pretence of smoking a bong.

Judge's brother, 24-year-old Steven Judge went too, although he was not part of the plan.

Benjamin Judge had packed a tool box, ropes, chains, a sledgehammer and tarp in the boot of the car before they left.

Once at One Tree Hill, the four shared a bong before Benjamin Judge tied a chain to a tree.

He then went at the victim with a sledgehammer and dragged him to the ground, where he and his brother assaulted him.

The victim was dragged towards the tree and the assault continued, while Dix yelled out, "You shouldn't have f***ed with the family".

County Court Judge Patricia Riddell found the victim was hit at least once with the sledgehammer, police having found his blood on its head.

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The victim tried to run but Benjamin Judge put him in a choke hold and he started to black out.

He feared he could die and took out a kitchen knife from his pocket before stabbing Steven Judge, who was on top of him, three times.

The victim ran into the bushes while Dix and Judge tried to seek help for Steven Judge.

Paramedics were called but he died at the scene.

The kidnapping victim was found hiding in a shed the next day.

He suffered multiple abrasions, bruises and lacerations to his head and body, including an injury that required stitches.

He was arrested, but police did not charge him over Steven Judge's death.

"This kidnapping is... a most serious offence," Judge Riddell said.

She said the impact on the victim had been significant, and Dix and Judge's moral culpability was high.

"Taking matters into your own hands in this way is unacceptable," she said.

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Judge Riddell said neither had shown remorse for the impact on the victim, but the death of Steven Judge had served as extra-curial punishment for them both.

Mr Judge's death, she said, also acted as a deterrent against future violent offending and should motivate their rehabilitation.

Judge Riddell also took into account their early guilty pleas, as well as their commitment to give evidence against two co-accused if required.

She noted both Judge and Dix had a history of significant mental health issues and suffered a difficult time in school due to bullying.

This was the first time in custody for both, and neither had prior criminal history.

Judge Riddell said Judge had good prospects of rehabilitation, while Dix's were "very positive".

Dix wiped away tears and turned away from the camera throughout the hearing, which was conducted via video link.

Judge and Dix have already spent 55 days in custody and must serve at least 22 months of their sentences before becoming eligible for parole.

Had they not pleaded guilty, they would have faced a six-year jail sentence.

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