Low-risk prisoners will have their jail time cut if they complete training and treatment programmes behind bars, under new National Party policy.

Under the "positive pathways" scheme, prisoners whose sentences are two years or less and who complete their training and treatment plan will be eligible for release 10 per cent earlier than under current settings.

The radical policy was announced today by Bill English and Corrections spokeswoman Louise Upston.

Prisoners serving more than two years will get a training and treatment plan from Corrections. Successful completion will trigger an earlier parole hearing. However, National said early release is not guaranteed and minimum non-parole periods will remain unchanged.

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How to address a record prison muster has become part of the political debate around law and order.

Today's policy is similar to one announced by Act leader David Seymour earlier this year, which would cut jail time if certain prisoners complete literacy, numeracy and driver licensing courses.

Tougher sentencing and bail conditions means the Government's books will be hit by an extra $2.5 billion over about five years as it builds new prisons and keeps more New Zealanders locked up.

Upston said rehabilitation programmes work and National wanted more prisoners to complete them.

"We will invest an extra $48 million in rehabilitation and reintegration programmes over the next four years to deliver another 6000 places. That's on top of National's major investment in rehabilitation, which has seen the number of offenders in training or treatment programmes almost triple since 2008."