He said the children affected were very vulnerable and the effects of solitary confinement in custody could be profound and unexpected.
"There is a real risk of self harm and even that a young person would take their own life."
Becroft himself had been on the front lines of such a tragedy.
"I had to conduct a coronial inquest where a 17-year-old boy who was said to be no suicide risk took his own life in a suicide-proof cell. It was devastating for everyone concerned.
"It brought home to me just how vulnerable and unexpected under 18s could be and that's not something that should be messed with."
Police Superintendent Chris Scahill said police recognised it was not appropriate for young people to be held in cells and were working closely with Oranga Tamariki to ensure that a police cell was an absolute last resort for a young offender.
"Police closely monitor these young people to ensure their wellbeing.
"Police have a responsibility to stop people harming themselves and others in the community. On occasion, this may mean that a young person must be held in a police cell for a short period of time."
Becroft, in alliance with Amnesty International, said legislation should be amended so that holding young people in police cells was not an option.
Under the Oranga Tamariki Act 1989, police cells were one of five options available to the Youth Court when deciding where to remand a young person pending their court hearing.
There's no limit on the amount of time a young person can be held in police custody under section 238(1)(e) of the Act.
Annaliese Johnston, advocacy and policy manager at Amnesty International New Zealand, said it was unacceptable that so many were spending several days in police cells, particularly before they had been found guilty of an offence.
"We are concerned that young people detained in police cells can be held in solitary confinement and without adequate food, lighting and hygiene facilities, 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."
Lack of beds in youth justice residences has been cited as one of the main reasons behind the high figures.
Johnston said Oranga Tamariki had been working towards reducing numbers by increasing capacity in suitable and safe community-based residences and some progress had been made.
"But the fact remains that law exists which allows for these placements to continue, which we consider to be an ongoing breach of our international human rights obligations."
New Zealand's international obligations, under the United Nations Convention on the Rights of the Child, required those under 18 were only detained in exceptional circumstances, and they have the right to be held in an appropriate custodial environment. A child's best interests and welfare must be a paramount consideration.
Johnston said seeing police cell placements at these levels raised serious questions about New Zealand's ability to be adequately child-centred.
"It's essential that we have sufficient community-based, safe placement options for our young people. It's time to do better.
Amnesty International New Zealand will be raising the issue at New Zealand's Universal Periodic Review before the UN Human Rights Committee in Geneva in early 2019.
