The low number of second warnings given in the area looked encouraging, said Mr Detlaff.
"Even if we keep one criminal behind bars who would have come out and committed further crimes, then we've done our job, we've made a difference to somebody who would have been a victim but now won't be," he said.
The Sentencing and Parole Reform Act 2010 came into force in June 2010, creating a three stage regime of increasing consequences for repeat serious violent offenders, according to the Ministry of Justice. There were 40 qualifying offences, comprising all major violent and sexual offences with a maximum penalty of seven years imprisonment or more. A first warning was issued when an offender aged 18 or over who did not have any previous warnings, was convicted of a qualifying offence. Once an offender had received a first strike warning, it stayed on their record forever, according to the ministry.
If an offender was convicted of a second qualifying offence they received a final warning and - if sentenced to imprisonment - would serve the sentence in full without the possibility of parole. On conviction of a third qualifying offence the court must impose the maximum penalty for the offence. The court must also order the sentence be served without parole, unless the court considered it would be manifestly unjust.