Holly Reed, 15, with her dog Toby. "They were my friends," she says. "I would like them to say sorry."

A gang of teenage girls urged a 15-year-old special needs student to flash her breasts and then filmed and shared the footage online.

The Wellington East Girls' College students pretended to befriend their schoolmate Holly Reed, who has Down syndrome, and encouraged her to perform tasks like licking the ground. They then shared the footage on Snapchat and Facebook with captions such as "slag" and "yummy".

The images were shared among students for months before the school and her parents found out. "It breaks your heart seeing things like that," said Holly's father Michael Reed.

Victoria University's professor Vanessa Green, whose research specialises in bullying and peer relationships.

​Experts said that despite nearly 100 people being convicted since the the Harmful Digital Communications Act was introduced, including at least five people being imprisoned, social media continues to be awash with cases like this. It's estimated one in five New Zealand children are bullied online.

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Victoria University's professor Vanessa Green said social media was the "perfect platform" for bullying. "Bullying is a group phenomenon. Without an audience, the bullying behaviour would cease. It's because people pass on the information or photos that this happens.

KEVIN STENT/STUFF Fifteen-year-old Holly, pictured with her dad Mike Reed, has been the victim of cyber-bullying.

"Everyone's looking for attention, for their moment. With bullying, that's exactly what they're looking for – status, to look more cool or popular. If belittling someone else will enable them to do that, they will."

Green has evaluated a bullying prevention programme, KiVa, which has been trialled in about 30 schools across the country. The programme was effective at reducing victimisation, she said, and showed the key to preventing bullying was changing peer culture early on. "Although a lot of people try to spend time talking to their children about being safe online, they might not tell their children how to be respectful to others online."

In Holly's case, the bullying was detected only after other students reported the images to the school.

GRAHAME COX/STUFF Martin Cocker, executive director of Netsafe, says online bullying is a major concern.

Mother Nicki Crawford said it had gone on over a period of months, and Holly had not been aware of what the girls were asking her to do.

Crawford said they chose to send Holly to the girls' school in Central Wellington, about a 30-minute drive from their home in Porirua, because they thought she would be supported. "We purposely chose an all-girls school because we thought it would keep her safe."

The school was doing the best they could, but only had information from students who had provided it, she said.

Holly said she was saddened by what happened. "They were my friends," she said. "I would like them to say sorry."

She said she didn't want them to do the same thing to anyone else.

At a meeting with some of the students, only one admitted they'd done anything wrong, father Michael Reed said. "The attitude of the girls was that: 'We're just teenagers, it's the dumb s... that we do'."

The girls involved, since being found out, had "disappeared" from Holly's social media and Facebook, he said.

Principal Sally Haughton said the school had taken the bullying very seriously.

They had been working on the situation for the past week, but were not yet sure how many students had been involved.

"Our deputy principals and our guidance teams have been working very hard to make sense of what's occurred and the information that's available to us."

At the meeting with families and students, there were some apologies made, she said.

As the school was in early stages of talking to students and families, it was too early to say what the consequences would be for the students, she said.

Netsafe chief executive Martin Cocker said New Zealand's youth population had an online bullying rate of about 20 per cent a year, or one victim in every five children.

He said there were four groups of people who suffered worst: anybody with a disability, minority ethnic groups, anybody who was not hetero-normative, and women. "Everybody in online safety knows if you meet those criteria, you're more likely to be bullied."

An issue which needed addressing was the speed which online bullying was deleted. "By the time something is reported and taken down, the person has experienced the harm of the bullying. The companies might do the right thing, but it might actually be too late to stop the harm."

Cocker's solution to reducing online bullying was a combination of education, regulation and enforcement, and industry response.

"In that regard, New Zealand is in pretty good shape. We have pretty good education programmes, a stronger regulatory regime that most countries. But still these things go through."

He pointed to the Harmful Digital Communications Act, established in July 2015, which set out rules for the first time on what constituted online bullying.

By October last year, 89 criminal charges had been laid, and seven people had been jailed under the act.

Another issue was exposure, he said. "Sometimes on social media, those people who would do the right thing aren't the group of people who see whats going on – parents, schools, law enforcement.

"It's an incredibly public forum, but often the actual group who sees the content don't feel they have the responsibility or right to do something about it."

Cocker said the students who reported the bullying should be lauded.

"If you think about it, just about society generally, we only have a few thousand police.

"The reason why most people don't break the law is because the community looks out for each other - our neighbours, our friends.

"We just need to do the same thing online."

WHERE TO GET HELP

Netsafe - 0508 638 723

Lifeline (open 24/7) - 0800 543 354

Depression Helpline (open 24/7) - 0800 111 757

Healthline (open 24/7) - 0800 611 116

Samaritans (open 24/7) - 0800 726 666

Suicide Crisis Helpline (open 24/7) - 0508 828 865 (0508 TAUTOKO). This is a service for people who may be thinking about suicide, or those who are concerned about family or friends.

Youthline (open 24/7) - 0800 376 633. You can also text 234 for free between 8am and midnight, or email talk@youthline.co.nz

0800 WHATSUP children's helpline - phone 0800 9428 787 between 1pm and 10pm on weekdays and from 3pm to 10pm on weekends. Online chat is available from 7pm to 10pm every day at www.whatsup.co.nz.

Kidsline (open 24/7) - 0800 543 754. This service is for children aged 5 to 18. Those who ring between 4pm and 9pm on weekdays will speak to a Kidsline buddy. These are specially trained teenage telephone counsellors.