The ACT party's "3 strikes" law has

[C_T_Russell]

three strikes clearly hasn't worked but I do feel there is still a need for some kind of escalating penalty for repeat offenders - maybe the sentence imposed is increased by 50% for each subsequent strike or some such, so 10 year minimum becomes 15 year minimum next time, 20 years after that etc...

Well with all these shootings in Auckland, something needs to be done.
Labour certainly appear to have been light on gangs, and national keeps campaigning how they will go tough on gangs and crime, yet they too have nothing to show for it when they were in power.
Keeping people in prison costs money too, so perhaps other ways to rehabilitate offenders, particuarly young and first time offenders may help.
Some countries use military style boot camps, etc.
Perhaps something like this should be explored here too.