Adult statistics graphed look similar:
The Department of Corrections has embarked on a major recruitment drive and aims to employ around 600 new Corrections Officers by September 2017, with at least 500 of them coming from New Zealand.NZ Lawyer recently published an article about the falling court statistics.
The new recruits are needed because the prison population is expected to reach 10,000 by 2017. This increase is due to more people being held in prison on remand than previously. Legislative changes have also meant prisoners serve more of their sentence in prison, and there has been an increase in prisoners serving longer sentences for more serious crimes.
Here's one explanation:
“The police have made greater use of pre-charge warnings and alternative mechanisms (eg, community and iwi justice panels) to ensure minor offending is more appropriately dealt with.”
I suspect that is the major reason. Not an improvement in behaviour. Serious offending continues at the same levels.