Labour says under National, the prison population will continue to rise faster than it ever has before.
Appearing on The Nation on Saturday, Labour deputy leader and corrections spokesman Kelvin Davis said projections done in 2015 predicted it would take a decade for the prison population to reach 10,000.
"That actually happened less than one year after that report came out."
Labour has promised to reduce prison numbers by 30 percent over the course of about 15 years. Part of that may come from reversing tough new bail laws, introduced after the murder of North Shore teenager Christie Marceau - who was killed by a teenager out on bail.
Mr Davis told The Nation many prisoners don't need to be behind bars - particularly those yet to be convicted, being held on remand.
"A number of people who are on remand in Mt Eden who have been there for say eight or nine months... they have said 'I'm just going to plead guilty because I'm sick and tired. I'm separated from my family... I'm just going to plead guilty.' There will be people on remand who are pleading guilty who may be innocent."
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He said rolling back some of the changes would help stem the rise in prison numbers.
"On current projections we're probably going to have 18,000 people in prison. That has to stop."
But National corrections spokeswoman Louise Upston, currently the minister, says every single person who is in Corrections' custody deserves to be there.
"Seventy-one percent are violent, [have committed] sexual offences or serious drug offending charges. We have the right people in prison at the moment."
The rest - which make up people convicted of dishonesty, property damage and antisocial offences - she says often have had a number of community sentences before ending up in prison.
"It actually takes quite a lot to end up in prison."
She says National has no plans to roll back the tough bail laws, even though they were costing the Government hundreds of millions of dollars in new beds and facilities.
"I don't think there's anything wrong with them and we don't have any plans to deal with them."
Mr Davis expressed support for ACT's policy of reducing sentences for prisoners who learn to read and write, and get their driver's licence.
Another idea Mr Davis presented was meeting with all the major parties' corrections spokespeople to "depoliticise" the issue and look at how other countries with larger populations manage to keep their prison numbers lower than ours.
"We've actually got to see what jurisdictions around the world are doing."
Ms Upston says prisoners are already getting education and training, but only inmates at low risk of reoffending would be considered for early release.