National inflated the impact of its sentencing and parole reforms by more than 400 prisoners while on the election campaign trail.

In releasing the policy, John Key said the twin measures of no parole for the worst repeat violent offenders and life in jail without release for the worst murderers would add up to 572 offenders by 2011 and probably require a new prison.

In October's campaign trail policy release, Mr Key said: "National estimates this policy could add up to a further 572 offenders to the prison population by 2011, meaning we may need to build a new prison."

However, this incorporated a 410 estimated increase the Justice Ministry had forecast to occur regardless of whether sentencing and parole policy changed. National's real estimate was actually 162 extra offenders.

Mr Key made no mention of the forecast figure being incorporated in the official release.

The up-to-date estimate of the policy's impact is even lower, with figures National released to the Weekend Herald yesterday showing it will add 50 offenders over the next four years.

The Weekend Herald questioned the 572 figure yesterday, after seeing information Corrections provided to its then Minister, Labour's Phil Goff, who inquired about the impact immediately after the release.

Corrections chief executive Barry Matthews told Mr Goff that the policy would affect "relatively few" offenders.

The policy, now in the Sentencing and Parole Reform Bill before Parliament, requires the offender to have committed two violent offences with a sentence of more than five years in jail.

On the second offence they lose the right to parole and must serve the full sentence. Mr Matthews said 40 more beds would be needed by 2011 to cover this.

He said the no-release policy triggered if prisoners had previously been sentenced to five years in jail for a violent offence would require 29 beds but over a period stretching out to 2026.

Asked yesterday how Mr Key could have used the 572 figure, a spokesman for Justice Minister Simon Power revealed that it included a forecast increase of 410 prisoners.

The spokesman said since becoming Government, National had got officials to revise the impact of the policies.

He said it was now estimated that 50 extra beds would be required after four years, with more gradually needed down the track as the impact of the murderers getting life without release kicked in.

If Act's three strikes policy was added, as it currently is in the bill, the officials estimated an impact of 70 extra beds after 20 years and 132 beds after 50 years.

The spokesman said National had been able to get limited data while in Opposition. "In any case, as a responsible opposition we thought it was better to over-estimate rather than under-estimate, because we wanted the public to be clear that parole changes come at a cost."

The spokesman said they incorporated the forecast increase because it would "be disingenuous not to".

The spokesman said National would still likely require a new prison because of the forecast increase.

* Sentencing policy will add up to 572 offenders to jail by 2011
- John Key, October 6, 2008.
* Sentencing policy will add 50 offenders over four years, with more gradually required after that
- National staff yesterday.