The drop may be due in part to increased adult diversion by the Police, particularly in the Auckland region. Some believe it is the result of remand prisoners spending less time in prison due to the reduction in deposition hearings.
There is no evidence that it is due to a reduction in the recidivism rate, due to increased rehabilitation programmes in prison. For that to happen, the programme increase would have had to occur prior to May 2009, as recidivism rates are usually calculated two years after prisoner release. I've asked for that information, but it's unlikely to be available at this time, and will not have been published at this point.
Have responded to SST's self-selecting poll on 3 strikes for young offenders. http://www.scoop.co.nz/stories/PO1106/S00253/three-strikes-for-younger-offenders-is-still-a-bad-idea.htm
Andrew BEcroft's paper 'How to turn a Child Offender into an Adult Criminal in 10 Easy Steps' is a good read. On www.rethinking.org.nz