Prime Minister Bill English has cast doubt on proposals by the ACT Party that prisoners should be rewarded and their sentences cut short if they learn to read, write, or drive.

ACT Party leader David Seymour is hoping to reduce re-offending by giving prisoners the skills to lead normal lives when they get out of jail.

The policy would not apply to the worst violent or sexual offenders.

Prime Minister Bill English likes the idea, but pointed out the government is already spending money on prisoner rehabilitation and the idea of cutting down sentences needs work.

Hey claimed the New Zealand public firmly believes criminals should be locked up for as long as possible.

English said the policy "is something we can look at".

Jane Maltby, Wellington president of the penal reform group Howard League, said it was comforting to see ACT turn towards rehabilitation but they believe the policy should apply to serious offenders who need more opportunities for education.

"What we need to be doing is looking at rehabilitative programs can reduce everyone's sentence," she said. "The entire time that they're in prison is going toward toward rehabilitation and not punishment."

The Howard League's chief executive Mike Williams, who is also a former Labour Party president, spoke at the ACT Party conference last week.

Government measures to ease the booming prison population will cost around $2.5 billion over the next five years.