Official figures have revealed there were 135 incidents of self-harm in youth justice facilities in the last five years.
In that same period in police cells there were seven self harm attempts, including suicide, by young people - one in 2016 aged just 13, police said.
Youth justice organisation JustSpeak director Tania Sawicki Mead said the figures were horrifying.
"We are talking about some of New Zealand's most vulnerable children who are in these conditions," she said.
"I think it's incredibly important to start thinking about what are the alternatives to being held in youth justice facilities or police cells."
Oranga Tamariki general manager of youth justice residences, Ben Hannifin, agreed that the figures were worrying but he was heartened by a reduction.
In 2014 there were 30 self-harm incidents, but by 2018 that number fell to 24.
Mr Hannifin said self-harm was often attempted by a few people repeating the same pattern of behaviour.
"The young people we manage ... they suffer," he said.
"Almost all the self harmers are young women, 100 percent of them are victims of violence, whether that be sexual or physical violence."
The young offenders were also dealing with all of the regular teenage struggles on top of their traumatic backgrounds.
Mr Hannifin said with that in mind, staff worked extremely hard to make young offenders feel safe and supported while in the residences.
While he admitted the residences themselves could be prison-like in their appearance, he said that was not what was causing the self-harming.
"You know there's concrete walls, there's steel doors, there's high fences so that's the correctional feel but the experience a young person has inside those walls isn't correctional, not by any stretch," he said.
"So I don't believe the environment of a residence is what's causing the self harm at all I believe the self harm we see in the residences is a result of the complexities the young person is coming from."
Children's Commissioner Judge Andrew Becroft said he was deeply concerned that seven young people had attempted suicide or self harm in police cells.
He has long advocated for a ban on detaining youth offenders in police cells post their first appearance in the Youth Court.
Judge Becroft said the cell incident involving a 13-year-old in 2016 showed how serious the situation was.
"Every young person I've ever spoken to, some of the toughest of the tough, in solitary confinement, in adult police cells, no natural light, poor sanitation facilities, they say it messes with their head," he said.
"We cannot predict in advance the serious effects it will have on well-being and mental stability."
A Nelson-based youth court lawyer, John Sandston, has long warned a teenager will commit suicide in a cell, and he said the figures validated that concern.
Police cells were a deeply dangerous place for adults, let alone vulnerable young people, he said.
In a statement, New Zealand Police said it was not appropriate for young people to be held in police cells.
It said officers worked closely with Oranga Tamariki to ensure cells were a last resort option, and said when detained, young people were kept separate to adult offenders.
Minister for Children Tracey Martin was contacted for comment on this story but declined to comment.
Where to get help:
Need to Talk? Free call or text 1737 any time to speak to a trained counsellor, for any reason.
Lifeline: 0800 543 354 or text HELP to 4357
Suicide Crisis Helpline: 0508 828 865 / 0508 TAUTOKO (24/7). This is a service for people who may be thinking about suicide, or those who are concerned about family or friends.
Depression Helpline: 0800 111 757 (24/7) or text 4202
Samaritans: 0800 726 666 (24/7)
Youthline: 0800 376 633 (24/7) or free text 234 (8am-12am), or email firstname.lastname@example.org
What's Up: online chat (3pm-10pm) or 0800 WHATSUP / 0800 9428 787 helpline (12pm-10pm weekdays, 3pm-11pm weekends)
Kidsline (ages 5-18): 0800 543 754 (24/7)
Rural Support Trust Helpline: 0800 787 254
Healthline: 0800 611 116
Rainbow Youth: (09) 376 4155
If it is an emergency and you feel like you or someone else is at risk, call 111.