The controversial three-strikes legislation will become law, after the National-Act Party bill passed its third reading in Parliament last night.

However, it came under repeated fire from Labour, the Greens and the Maori Party in an at-times colourful debate, in which the substance of the bill and its process through Parliament came under scrutiny.

The bill has been criticised as having little impact on crime.

Legal experts have said all strike three convictions for an offence will be treated the same, discounting the differences in the severity of the crime.

Act MP David Garrett this morning said he thought a 10-20 per cent decrease in crime because of the law was realistic.

"I don't have a crystal ball. I can't say whether it will be 10 per cent, 20 per cent, but I would expect a marked reduction once the policy is fully in place."

He said the policy worked through "incapacitation and deterrence".

"People cannot harm members of the public when they're in jail," Mr Garrett told Radio New Zealand.

"This law is aimed at that small number, that very small number, of repeat violent offenders who rack up cricket scores of offences which will no longer be possible."

Maori Party MP Hone Harawira said the law would not reduce crime at all.

"Just by putting a guy in jail for the rest of his life doesn't stop the fact that other people are in the same conditions and are likely to start heading down the same path."

Nothing has been done to change the conditions.

During the debate last night Mr Harawira said the policy was "outright bloody racism" against Maori.

Corrections Minister Judith Collins, who is in charge of the bill, yesterday dismissed any suggestion that a conviction under the strike offences could be for something minor.

"This bill deliberately puts in place an escalating regime of penalties, and I make no apology for that. An offender who is being sentenced at stage three has committed a third serious violent offence and has received two previous warnings about the consequences of re-offending in this way."

The bill also allows courts to order a life sentence without parole for the worst murders, even if the offender has no previous convictions.

The Greens' David Clendon, in opposing the bill, referred to the work of Auckland University's Professor Warren Brookbanks and Richard Ekins, who have said that offenders on strike three will have no incentive to plead guilty, and likely drag victims through the court process and increasing legal costs.

Labour's Ruth Dyson dismissed the bill as drivel that had been championed by Act MP David Garrett, a staunch supporter of the Sensible Sentencing Trust.

"We have sent a message to repeat violent offenders," Mr Garrett said.

"They have a choice - they can change their ways or they condemn themselves, by their own choice, to spend most of their lives in jail."

National and Act had sufficient numbers to pass the bill 63 votes to 58 (the Progressive Party did not cast a vote), prompting applause from both Act MPs, and Sensible Sentencing Trust members in the public gallery.

Labour MP Parekura Horomia called out "what a disgrace", as the bill was passed.

How it works:
40 qualifying offences, ranging from murder to indecent assault to robbery. Conviction is an automatic strike.
* Strike one: Judge decides sentence, gives a warning, offender eligible for parole.

* Strike two: Judge decides sentence. No parole.

* Strike three: Maximum jail term for that offence. No parole, except for manslaughter (minimum non-parole period of 20 years).

- With NZPA