A prison watchdog is investigating how young people are treated in custody.

The Office of the Inspectorate is examining the conditions and experiences of young inmates, with one in 10 prisoners under 25 years old.

The inspection, which began in September, will assess whether Corrections is acting in line with New Zealand law and international best practice.

Inspectors will speak to young people at each of the country's 18 jails, as well as their families and staff.

Chief inspector Janis Adair says changes to justice laws and policies in the last five years had affected young prisoners - including a 2019 update that saw 17-year-olds included in the youth jurisdiction.

Other significant events, including the closure of Hawke's Bay Regional Prison's youth unit after a riot, has also impacted how young people are managed in prison, says Janis.

"On average, approximately 80 per cent of young men in Corrections' custody remain accommodated in mainstream facilities rather than the youth units, mostly in Mount Eden Corrections Facility and Spring Hill Corrections Facility," she said in the inspection's terms of reference.

"Given the small number of females under the age of 18 years and under the age of 20 years, there has never been a dedicated youth unit in a women's facility, meaning they are placed in a mainstream facility."

Research over the past two decades has established young people typically don't fully transition to adulthood until their mid-20s, says Janis, and the investigation will look for examples of good practice and areas for improvement.

"The purpose of this inspection is to investigate how Corrections considers and responds to the unique developmental needs, challenges and opportunities for young people under 18 years old and young adults aged 18-25 years in its custody.

"The inspection will also be informed by evidence of custodial environments and practices that cause immediate and long-term harm to young people and young adults in adult custodial facilities as well as effective practices for this population if in custody that improve behaviours, attitudes, and long-term outcomes."

The findings are due to be published mid-year.

-Soumya Bhamidipati/RNZ.