Claims of fight clubs and staff punching young people at a youth justice residence are detailed in a new report by the Children's Commissioner.
The commissioner, Judge Andrew Becroft, said while the latest State of Care report found no evidence of systemic abuse of young people in either youth justice or care and protection residences, he was worried by an undercurrent of violence and bullying and wants more inspections of the centres.
"Whenever we group violent young boys together, there is the possibility and indeed the likelihood of bullying and violence," Judge Becroft said.
"We know that goes on, the boys who were there said, 'yep, it happens, but snitches get stitches, we sort it out ourselves'."
Part of the problem was that young people were less likely to complain about serious issues like bullying or physical abuse from other young people or staff members because they were scared about the consequences, the report said.
The Office of the Children's Commissioner also uncovered allegations of a fight club at one youth justice residence, with one young person commenting: "There are fight clubs and staff punch young people in the body where it won't mark; they do it away from the cameras".
This was brought to the attention of Child, Youth and Family, which has since been replaced by Oranga Tamariki, and the findings of a subsequent investigation were inconclusive and the claims remained unsubstantiated.
But Judge Becroft said he remained anxious about the possibility of that sort of thing happening and New Zealand could not be complacent.
He said he wanted his staff to be able to inspect residences much more frequently - every six months rather than every 12 to 18 months - including more unannounced visits.
He would also like to be able to send in investigation teams as quickly as possible if there were allegations about a serious incident.
But to do that, more funding would be needed, Judge Becroft said.
Oranga Tamariki chief executive Grainne Moss said the ministry treated allegations of violence in secure residences very seriously and they were thoroughly investigated, whether it related to staff or young people.
She also said the ministry was working to strengthen the complaints process so young people could notify staff about bullying and abuse safely.
The State of Care report focuses solely on the country's five care and protection and four youth justice residences.
It was prompted by revelations last year about abuses at a youth detention centre in Australia's Northern Territory.
Judge Becroft said he wanted to be sure that that could not happen here.
While there was no concrete evidence of systemic abuse or inhuman practice, Judge Becroft said the standard of care and conditions across the residences was far too variable.
"The report shows the overall performance of both care and protection and youth justice residences is middling and there's room for significant improvement.
"There are some pockets of excellent practice, but also sub-standard practice," he said.
Parallels with violence in adult prisons - lawyer
Lawyer Sonja Cooper, who specialises in abuse in state care, said it was concerning that similarities could be drawn between the alleged violence at youth justice residences and the country's adult mens' prisons.
Ms Cooper told Morning Report that although there was no "concrete evidence" behind the claims, it was clear there needed to be far more monitoring of the facilities.
"I think it's concerning that the young people say staff are hitting them where the marks can't be seen; fight clubs are clearly endorsed, or at least not stopped, by staff; girls talking about ... being beaten up by other girls as the staff stand on.
"These are sorts of things that we've heard over and over again - and to hear them being reported still in 2017 as being part and parcel of a youth justice centrev or a care and protection residence is deeply disturbing."
She said introducing random inspections and providing vastly greater monitoring was essential.
"One of the things that Judge Becroft suggests is that there are random inspections that can take place. I absolutely agree with that.
"There should be inspectors who can investigate complaints.
"We already ahve a process under teh mental health system - we should be copying that for young people.
The report covered the period between July 2016 and March 2017, before Child, Youth and Family was merged into the new Ministry for Vulnerable Children, Oranga Tamariki.
The Children's Commissioner monitors the residences as part of New Zealand's obligations under the United Nations Optional Protocol to the Convention Against Torture.