Corrections Minister Judith Collins has expressed concern at suggestions by the prison officers' union that prisoners should be released into the community because it will be too dangerous to keep them in prison.

Ms Collins was responding to comments by the President of the Corrections Association of New Zealand, Beven Hanlon, that the new three-strikes law would result in prisoners harming guards because they had nothing to lose.

Three strikes will see the country's worst violent repeat offenders serve out maximum sentences without parole on their third strike. For offenders convicted of murder, that could mean they spend the rest of their lives behind bars.

"I am astonished that the union would suggest that prisoners they regard as too dangerous to be kept in prison be released back into the community through shorter sentences," Ms Collins said.

"Our Corrections Officers do a great job of managing the country's most difficult and dangerous people, and their safety is a priority. These staff are professionals who are fully committed to keeping the rest of the community safe from these offenders.

"I believe it sends the wrong signal when their union starts making excuses for these hardened criminals.

"While Mr Hanlon might want to give them an easy ride, this Government is not prepared to go soft on those repeat violent offenders who might choose to misbehave once in prison."

Ms Collins said the three strikes legislation would keep the worst offenders in prison for longer, use escalating sentences to deter criminals from further offending and bring certainty to sentencing for the very worst offenders.

"This Government is serious about better meeting the needs of the victims of crime. This Bill and its three-strikes provisions is an acknowledgement that the justice system exists to serve them, rather than offenders," Ms Collins said.