National has vowed to abolish parole for some violent repeat offenders under its proposed "life means life" policy.
Party leader John Key says the proposal would require $314 million for a new prison and an extra $43 million in running costs.
Mr Key says that under the policy, offenders would have to serve their sentences in full if they have already been convicted of a violent crime and sentenced to five years or more in prison.
He said he does not want such offenders roaming around the streets.
However, Mr Key says the policy still gives a second chance to offenders, who will get one chance at parole.
He says the cost of the policy is affordable compared to the cost of letting violent offenders back on the streets.
But the Howard League for penal reform says National's plan is a backwards step to the days of hanging.
League president Peter Williams QC, says if an individual cannot get parole there is no incentive for him or her to reform.
Mr Williams says there's no evidence to suggest longer sentences reduce crime.
But the Sensible Sentencing Trust says National's plan will take away a huge amount of anxiety for offenders' victims.
Trust spokesperson Garth McVicar says the announcement is good news for victims of violent crime.
Mr McVicar believes such a policy would also deter people from committing violent crime.