A teenage victim at the centre of one of New Zealand's most aggressive cyber attacks will fly to Denmark to give evidence against the man who allegedly tried to exploit her.

The man's trial comes more than three years after he allegedly hacked computers at the girl's Auckland home, her school and her father's business in revenge for a failed online relationship.

He was arrested in a joint operation between New Zealand and Denmark in September 2015, and charged with 12 crimes, including sexual harassment, hacking and child pornography.

The case came to light after students at an Auckland school complained about a denial-of-service attack on the school's IT system.

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It lasted several weeks, with experts and police called in to help.

Authorities later revealed girl's relationship with the man began through online gaming when she was aged 15. It allegedly progressed to sharing explicit images, and then Skype calls where the girl undressed in front of a camera.

When she wouldn't provide more images, he is alleged to have attempted to blackmail her into providing more pictures, by threatening to post the original images to the web.

When that plan failed, the man is claimed to hacked her family's home computers, stole their information, and then launched a denial-of-service attack on the servers of the school, rendering them useless.

He also allegedly uploaded the explicit images and videos of the girl to pornography websites and bombarded the school's Facebook page with links to the sites, in an attempt to shame her.

"He is trying to punish the child or the family. It's a jilted relationship. He wants revenge," the principal told the Herald at the time.

The pressure on the teen was immense, she said, and she had to undergo counselling to cope.

The school held assemblies to warn them about the dangers of sharing images online, and also sent letters home to parents to explain the situation.

Police said at the time it was one of the most aggressive attacks they had seen, calling it a "particularly distressing experience for the victim and her family".

The girl and her mother will travel to Denmark to give evidence at the trial next week.

New Zealand police officers, employees from the father's business, from the school, and from Vodafone will give evidence by Skype - alongside Danish police and tax officials, as the man is also charged with tax evasion.

The 27-year-old man is unlikely to go to prison, however, as he has been diagnosed with a personality disorder.

Instead he can expect some sort of forced mental treatment.

The trial begins on Tuesday.

The school's new principal said as the incident happened before he started there he did not have full knowledge of the case and couldn't comment.

A New Zealand Police spokeswoman confirmed one of its officers would give evidence at the trial.

Police would not comment further.