A teenager motivated by greed who sent "evil and sinister" letters to parents of Christchurch primary school children, threatening to harm them if thousands of dollars in cash wasn't handed over, has today been sentenced to five months home detention.

The 17-year-old Christchurch boy, who has been granted permanent name suppression, along with his victims and the school, put threatening letters in the letterboxes of families living in Christchurch on June 6 and June 14 last year.

The notes told the parents he knew where they dropped their kids off at school.

If they did not pay sums varying between $5000 and $10,000, their children, or other pupils, would be subjected to serious violence.

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A second letter increased the extorted amounts.

Three months later, police caught up with the youngster after he was spotted on surveillance cameras, and he was charged with extortion.

The teen pleaded guilty at Christchurch District Court.

Today when he was sentenced, the court heard how a "bright young man" who plans to study commerce at university had been bullied and socially isolated when he hatched his plan, branded an act of "total stupidity".

The crimes were driven by personal greed and a desire for money by the boy whose fledgling business was failing, the court heard.

However, defence counsel Richard Maze said he never intended to carry out any of the threats.

His demands, called "wholly selfish" by Mr Maze who added that they defied rational or reasonable explanation, were ultimately unsuccessful.

The bizarre threats were "in large part caused by a total lack of empathy", Mr Maze said.

He lacked the ability to have any understanding the effect his actions would have on either the victims, the community he targeted, or indeed his own family who were in court to support him today.

The boy had been bullied and socially isolated and had changed schools, the court heard.

Since the offending, his grandparents paid for a 10-day adventure course where he "flourished".

It was "highly unlikely" that he could cope in prison, Mr Maze said.

His age was a mitigating factor, Mr Maze said, citing evidence that youth have poor judgement but a high ability to be rehabilitated.

Judge Brian Callaghan described the offending as "evil and sinister".

"One can only imagine, as a parent, the immediate repercussions emotionally and psychologically when harm is threatened to those they are nurturing and bringing up," Judge Callaghan said.

"One cannot gloss over the seriousness of the initial impact of these matters."

A restorative justice meeting was held between the offender and his victims.

The judge said this case highlighted just how important such conferences can be for the victims who need to try to understand the offending and to hear the offender acknowledge his regret.

The victim's parents had been "charitable, benevolent, understanding and compassionate" throughout the process, "in a truly moving way", Judge Callaghan said.

They wanted to support the defendant in his rehabilitation and also supported the name suppression application.

Judge Callaghan sentenced the teen to five months' home detention and ordered him to pay a "token" reparation payment of $900 to each of the two victims. He was also ordered to carry out 160 hours of community work.

He also made a final suppression order, banning publication of the identity of the defendant and his family, his victims and their families, the school and the area they are all associated with.