Using cutting-edge data modelling to help decide criminal sentences is worrying for some.

The Government has approved spending over a million dollars on a modelling system, to indicate which sentencing options will reduce the likelihood of re-offending for people with different ages and criminal histories.

LISTEN ABOVE: Justice Minister Amy Adams speaks to Mike Hosking

Justice Minister Amy Adams said it's almost amazing that the current system relies on societal instinct, rather than an analytical system.

She said it could also save millions of dollars, by predicting which sentencing option will most reduce the chance of re-offending.

However, Labour's justice spokesperson Jacinda Ardern believes data can't take everything into account.

"I think we have to be so careful about trying to extrapolate out or predict someone's behaviour from raw data rather than what we have in front of us about that individual person and their individual circumstances."

Ardern said the data alone will not be able to provide a big enough picture.

"How they might respond to restorative justice, what kind of family support networks they have because we know that's a key player in whether someone is properly able to rehabilitate."