Justice Minister Amy Adams says government agencies must change their approach from "picking up the pieces", to making earlier interventions to reduce crime.

Tackling health and education issues facing vulnerable Kiwi kids is the best way to keep them out of the country's prisons, Justice Minister Amy Adams says.

A new approach to New Zealand's justice system could also lead to judges making sentencing decisions based on data analysis of the most effective approach.

Adams launched the "investment approach to justice" on Monday morning, saying it was "quite a revolution" in the way the sector worked.

CHRIS MCKEEN Finance Minister Bill English is leading a push for more data-driven decisions on public spending by the Government.

Accounting giant PWC will help the Ministry of Justice develop statistical models to predict lifetime patterns of offending and victimisation, and help forecast future crime.

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The project, which has received $2 million of funding, will use data from Statistics New Zealand's Integrated Data Infrastructure, which holds New Zealanders data collected by a number of government agencies.

The launch is part of the Government's "social investment" approach spearheaded by Finance Minister Bill English, aimed at making better spending decisions data on vulnerable Kiwis.

Adams said the focus was on stopping people from committing crimes, as much as dealing with them once they were in front of a judge.

"By the time they're in court, it's a much harder life course to change around, so if all of the focus of this is what we can sentence people to in court, we're really missing the point."

JUSTICE SECTOR 'NOT AN ISLAND'

Most of the initiatives she now lobbied for as justice minister were in areas outside of her portfolio, but which were most likely to tackle the causes of crime, such as literacy and mental health.

"The justice sector is not an island - these people don't suddenly turn up one day in the justice system, and our best opportunity to stop them committing crime happens long before they come to us."

While only 11 per cent of the New Zealand public used mental health services, 40 per cent of those charged in court - and 51 per cent of those starting a community sentence - had received help for a mental health issue.

An early "investment brief" also showed strong evidence that cognitive behaviour therapy helped to reduce crime, as one out of every five to 15 people would not reoffend after receiving treatment.

However, Adams said the Government had to balance the initiatives with the need for "punishment" as part of the justice system.

"We can keep punishing people for horrific acts, but I would also like to stop these horrific acts having occurred, and that's what this is about."

DATA ANALYSIS FOR JUDGES

An early piece of analysis showed that people fined for offences like assault, shoplifting and drink-driving, were less likely to reoffend and access benefits than similar people sentenced to community service.

While judges were entirely independent and had ultimate discretion over sentences for criminals, Adams said they had similar motivations to reduce reoffending which the data analysis could help.

While the Government didn't have "vast amounts of money to throw at new things", Adams said it could look more critically at the initiatives it was funding to understand which were making a difference and which should be scrapped.

"The Government spends billions of dollars every year on supporting our most vulnerable households and individuals, but for much for that spending, we simply don't know what is making a difference."

'MONEY AT BOTTOM OF CLIFF NOT WORKING'

One challenge was the "lag time" between introducing earlier interventions and any results, but it was clear that government agencies needed to change their approach from "picking up the pieces".

"It isn't...an overnight fix, but it is quite a revolution in the way the justice sector, both itself and within government, have worked.

"There is a complete commitment across Cabinet ...to say that continuing to put money at the bottom of the cliff is not working for us."

Adams said the first "big wave of data" from the project was likely to arrive towards the end of the year.