A criminal justice advocate believes bringing back juvenile detention centres is a great idea in theory, but not in practice.

Act Party has unveiled a policy promise to invest in 200 new beds in juvenile detention as part of the Alternative Budget it's announcing next week.

Sir Kim Workman told Mike Hosking history tells us what happens when youths are institutionalised.

He says when he was a police officer young people would be released just to end up quickly back in institutions or prison.

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