Key Points:

Corrections Minister Phil Goff says National's life-means-life parole policy for murderers is just "for show" and would pointlessly and expensively keep geriatric convicts in prison.

Mr Goff said murderers who already had serious violence convictions such as RSA triple-killer William Duane Bell and prison van basher George Charlie Baker would likely be locked away forever under existing law anyway. He said if they did get out, they would be too "old and infirm" to be of any risk.

The life-for-life policy is part of National's plans to abolish parole for serious repeat offenders. For instance, if an armed robber sentenced to seven years had a previous serious violence conviction, he would have to serve the entire seven years without question.

Mr Goff said this would mean keeping a convict in prison even if they had been deemed not be a risk to the community.

"I can see no point in spending money to create a public impression," Mr Goff said.

He cited the American experience of prison rooms "full of geriatric people", which came at huge expense but did little to improve the safety of the community.

"As a person becomes geriatric, the conditions under which you have to keep that person and the care you have to give them becomes much higher. It actually becomes more expensive, with less return to the public in terms of public safety."

Mr Goff said people in their 80s "are generally not going to be very good at committing crimes".

National says its life-means-life policy would have applied to 10 out of 144 murderers since 2002.

Abolishing parole will have a bigger effect when other violent crimes are taken into account, and National estimates this will mean an additional 572 offenders in jail by 2011. It has estimated a new prison costing $314 million will be required, as will extra costs of $43 million a year.

Mr Goff said National was "pretending" by saying these would be the only extra costs needed, as prison numbers were already forecast to rise. He said four new prisons and 2300 extra beds have been provided over the past five years, but had still not slowed the growth.

Act leader Rodney Hide said the policy was "cynical politics" and only introduced really tough sentences when someone was murdered.

Mr Hide said this would not actually prevent the most serious crimes, unlike Act's "three strikes" policy, which would see offenders locked up for 25 years after they committed three violent offences.

NZ First law and order spokesman Ron Mark called the policy "hot air electioneering".

Mr Mark said National had consistently voted against the NZ First project to get 1000 extra police on the streets, and said voters should look to "who delivers" rather than the "usual limp-wristed policies offered by the old parties".