Data from Oranga Tamariki showed in the year ended June 2022, 42 children and teenagers were kept in a cell for more than one day.
At the end of June 2024, that number was 418.
Youth advocates and lawyers told RNZ that was due to a lack of available beds in youth justice facilities and police being more likely to prosecute youth offenders.
Tauranga lawyer Rachael Adams told Nine to Noon her 14-year-old client almost spent a second night in police cells recently, until a Youth Court judge intervened.
"A very troubled girl with a background of quite complex trauma. The offending isn't in itself very serious, it involves a number of shoplifting charges, quite a shoplifting spree.
"The difficulty was trying to get this young woman to engage with any sort of supports or restorative process or any sort of Youth Court initiative."
After her arrest they were told there was no place in an Oranga Tamariki residence anywhere in the country, Adams said.
As a "last resort" she spent a night in police cells
"The police are hardly comfortable about a disturbed, unhappy, traumatised young woman in their care so they kept her under close observation for her own safety, what that means of course is there's no privacy."
In court the following day, the judge ordered her to be remanded in Oranga Tamariki custody, meaning they had to find a place for her.
Which they did, in Palmerston North - a long way from her family.
"It's not the first time, in my experience, that has happened. It takes a brave judge to force Oranga Tamariki in this way.
"Sometimes this means that a social worker has to physically guard the young person that night, either at a motel or worse in Oranga Tamariki offices. That's a horrible obligation to place on a social worker, it's not safe, but that's that's happened too."
Adams said she wasn't surprised about the rising statistics. She believed it was due to the increasing seriousness of youth crime, and a lack of resources at Oranga Tamariki.
"The saddest thing, in an increasing number of these cases, is that inevitably we're just going to put band aids on things until it's serious enough, or they become old enough, to be elevated to the District Court."
Principal Youth Court judge Ida Malosi said judges were concerned about having to remand people into police cells.
However, she said there were "checks and balances" in place for young people held in police cells, including the need for a review by the court every 24 hours.
She also got a daily report about young people being kept in police cells.
"I have very good lines of communication with both the police and Oranga Tamariki and I have always found them to be immediately responsive to cases drawn to their attention.
"Taking into account all of the traumas that they inevitably bring with them, no one would argue that their needs can be met in a police cell."
Holding young people in police cells should be reserved for the most serious of offenders where there was a risk of violence, or them absconding, Malosi said.
Oranga Tamariki national director youth justice services and residential care Iain Chapman said the latest statistics highlighted a trend over the last seven years.
Young people were being given longer remand sentences, the number of people in youth justice facilities was back to pre-pandemic levels, and the seriousness of offences was increasing again post-pandemic, he said.
"All of this adds to demand for spaces in youth justice facilities. Police custody episodes have increased, and this demand is part of the reason why."
Chapman said they aimed to move children and young people out of police custody as soon as possible.
"Our teams assess, review and prioritise placements every day."
