The proposed "three strikes and you're out" sentencing policy could breach human rights, the Attorney-General says.
The Government agreed to support the three strikes policy through its first reading as part of its support agreement with ACT.
It was included in the Sentencing and Parole Reform Bill which has gone to a select committee for consideration.
In his report to the house, Attorney-General Chris Finlayson said the policy "may raise an inconsistency with the right against disproportionately severe treatment".
Section 9 of the Bill of Rights gives everyone the right "not to be subjected to torture or to cruel, degrading, or disproportionately severe treatment or punishment".
The policy would see the sentencing court given a choice between a sentence less than five years or a life sentence for someone committing their third serious offence, Mr Finlayson said.
"As a result, the scheme does not ensure a consistently rational connection between the offence and the penalty."
A "strike offence" was defined as the most serious violent and sexual offences, including murder, attempted murder, grievous bodily harm, serious firearms offences, rape and a range of sexual offences on children and young people.
On the third strike the offender would get mandatory life imprisonment with a minimum non-parole period of 25 years.
Mr Key said the Government was reserving judgment on whether it would support the policy further.
Labour, the Greens and the Maori Party all opposed the bill.
