The "three strikes" law could see some of the worst criminals still eligible for parole depending on the order in which they commit their crimes, a criminologist has said.

The law, which National and Act have agreed to pass, opens the way for huge inconsistencies in sentencing, said Professor Greg Newbold.

The University of Canterbury criminologist said an offender who committed two assaults and a murder - in that order - would be locked up for life, because the maximum sentence for murder was life imprisonment.

But the new law would mean that someone who committed murder and then two assaults would only serve the maximum penalty for assault, a sentence length that varies depending on the attack and not a life sentence.

Police Minister Judith Collins last night defended the law, saying most murderers were given life sentences in which they were eventually allowed to apply for parole, which carried set conditions.

"If the offender later reoffends when on parole then they will most likely be recalled to prison to continue to serve their life sentence for the murder [first strike] as they have breached the terms of their parole."

But Professor Newbold said a prisoner would still be able to apply for parole within 12 months of being recalled to prison.

He said the law was "stupid" and lacked credibility and predicted it would be challenged by defence lawyers.

Under the three strikes law, once an offender is convicted of a third serious offence the judge will have to impose the maximum sentence for the crime.

The offender will not be eligible for parole while serving time for their third offence.

Professor Newbold believed sentences should be left to the discretion of the judge and the release date of prisoners left to the discretion of the Parole Board.

He said prisons will be made even more dangerous by the three-strikes law and prison guards could find themselves targets.

"If you're doing life without parole, why wouldn't you kill a prison officer? What's to stop you? What would you lose by killing a prison officer?"

Howard League for Penal Reform president Peter Williams, QC, said some criminals with a record may kill in order to avoid being caught by police.

"If people realise that if they are going to be arrested, they are going to be in there [prison] for their natural life, there may be circumstances where they are going to kill," Mr Williams said.

Mr Williams said prison officers would have few deterrents when dealing with prisoners serving life sentences once the law went through.

"I wouldn't envy the lot of prison officers keeping these people," Mr Williams said.

Mrs Collins said preventive detention would be retained.

Preventive detention would give offenders the opportunity to be released after a shorter term than life without parole and the policy gave the worst repeat offenders certainty that the sentence they received would be harsh, Mrs Collins said.