More young people aged under 18 are being held in police cells and they are being kept there longer.
According to data released to Amnesty International, in the year to March, 165 under-18s were held in cells for an average of 2.6 days.
In June 2014, the equivalent figures were 62 and 1.8 days.
The number of those being held has increased by 166 percent over the past four years.
Amnesty International advocacy and policy manager Annaliese Johnston said holding under-18s in police cells should be illegal.
"We consider [it] to be an ongoing breach of our international human rights obligations," Ms Johnston said.
"Amnesty International is recommending that legislation be amended to remove the option to remand a young person in police custody, and will be raising the issue at New Zealand's Universal Periodic Review before the UN Human Rights Committee in Geneva in early 2019."
She said it was concerning that young people could lawfully be held in cells - including solitary confinement - without adequate food, lighting or hygiene facilities while in close proximity to adult prisoners.
"Extended detention in these conditions can lead to long-term physical, mental and emotional harm and the very real risk of self-harm.
"Detention in a police cell for multiple nights is no place for a child.
"It's unacceptable that so many are spending several days in police cells, particularly before they have even been found guilty of an offence."
Ms Johnston said a lack of beds in youth justice residences was one of the reasons for the rise.
Police response and operations national manager Superintendent Chris Scahill said a police cell for a young offender was a "last resort".
"Police have a responsibility to stop people harming themselves and others in the community.
"Police recognise that it's not appropriate for young people to be held in cells and we work closely with Oranga Tamariki to ensure that a police cell is an absolute last resort."
Mr Scahill said it was inappropriate for the police to comment on any proposed law change.