The prison gate has become a revolving door and changes have to be made to how prisoners re-enter society, a Salvation Army report says.

Author Annaliese Johnston says many inmates, instead of being effectively rehabilitated and reintegrated, become "repeat customers".

The report - Beyond the Prison Gate: Reoffending and Reintegration in Aotearoa New Zealand - argues that the system is failing prisoners and the public.

Ms Johnston says the cost of prison is spiralling out of control and doing nothing to reduce reoffending.

She says refocussing efforts on effective alternatives and reintegration would save the country millions, reduce crime and make the country safer.

"As one of our research participants put it, 'Instead of the government spending over 90 grand to keep us in prison every year, why don't they invest it on keeping us out?'"

Ms Johnston said statistics show that, despite crime rates falling overall, the prison population had tripled since 1985.

Corrections was unlikely to meet its composite target of reducing reoffending by 25 per cent by 2017, and post-prison reoffending rates had barely changed in the past 10 years.

The report, to be launched on Wednesday, recommends reducing spending on custodial prison sentences.

Instead, it suggests moving investment to programmes that will reduce crime and support reintegration for released prisoners.

Other recommendations include better access to accommodation and health services, private-public partnerships to provide employment, and mentors.