A criminal defence lawyer believes locking people up for longer won't necessarily make communities safer.

Thousands more offenders are likely to be captured under the Three Strikes rule, following changes by Cabinet to toughen up the new regime.

It's lowering the threshold for a first strike warning from 24 months imprisonment to 12 months.

Roderick Mulgan told Ryan Bridge offending is complicated.

He says it's not cause they're bad people, they offend because of things like substance abuse and lack of family support.

Mulgan says the idea of putting people away and forgetting where you left the key,  is a bit old fashioned.

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