The proposed "three strikes and you're out" law could lead to such severe punishment it would breach fundamental human rights of New Zealanders, according to the National Government's own legal advice.

Attorney-General Chris Finlayson has found the three-strikes bill has an "apparent inconsistency" with the section of the Bill of Rights protecting New Zealanders against cruel, degrading or "disproportionately severe" punishment.

Three strikes would see those convicted of a third serious offence sentenced to life imprisonment with a 25-year non-parole period.

It is a key Act policy and has been introduced to Parliament by National as a condition of the minor party's agreement to support the Key Administration - with National reserving judgment on whether it will support the legislation further.

As Attorney-General, Mr Finlayson is required to report any bill that appears inconsistent with the Bill of Rights.

His concerns relate to the inconsistencies it would lead to, such as "the imposition of a life sentence for offences that would otherwise be subject to a penalty of as little as five years".

Mr Finlayson said this could lead to a sentencing judge effectively being left with a choice between a sentence of less than five years or a life sentence.

It could also lead to sentencing judges having to impose significantly more severe sentences on an offender on his third strike than on a more culpable, but non-qualifying, offender who committed a similar crime.

Mr Finlayson said the bill did not reflect the differences between an offender whose previous offences happened in the "distant past" and one who committed three crimes in quick succession without gaining convictions that would make him or her eligible for the three-strikes penalty.

He pointed out the existing provision for preventive detention already allowed for certain offenders to be kept in prison forever and applied to almost all of the three-strikes offences.

Mr Finlayson said the legislation could result in "disparities between offenders that are not rationally based" and "gross disproportionality at sentencing", which raised the apparent inconsistency with the Bill of Rights.

The minister drew the conclusion even after noting a Supreme Court ruling that the particular section of the Bill of Rights could be invoked only when the punishment complained of reached "the very high threshold of outrageousness".

Mr Finlayson's report was done in his capacity as Attorney-General and is not his opinion as a National MP and minister. It is based on advice from the Crown Law Office.

The three-strikes provision was included in National's Sentencing and Parole Reform Bill, which has gone to a select committee for consideration.

* Know your rights
What three strikes would do:

Criminals convicted for a third time of a serious violent offence will be sent to prison for life with a minimum non-parole period of 25 years.

What the Bill of Rights says:

Everyone has the right not to be subjected to torture or to cruel, degrading or disproportionately severe treatment or punishment.