A family's concern over school bullying led to two teenagers being subjected to violent, bizarre, and depraved vigilante justice.

A 40-year-old man, who has name suppression, admitted charges of kidnapping, sexual violation and assault today at Christchurch District Court.

The court heard how three brothers illegally detained two teenagers on June 8, 2015, after allegations of a "physical altercation" and bullying.

The victims, both aged about 16, were leaving Riccarton Mall in Christchurch when they were confronted by three men, the court heard.

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They were told to get into a car as they were going to go and see their parents and "sort out what happened".

When the teens said they would follow them, one of the men told them: "No, you need to come with us."

They had their phones taken and were driven to a park outside Christchurch where they were beaten around the head and body. One of the youngsters had an ear-ring ripped out.

One of the victims had their pants pulled down before they were sexually violated.

Judge Raoul Neave today remanded the 40-year-old on bail to be sentenced for his role in the attack on May 16.

His two brothers – aged 28 and 33 and who also have suppression – have already been sentenced to six months' community detention and community work.