One prisoner only - a recidivist robber - has earned the infamy of having a second "strike" to his name since the introduction of the three-strikes system for sexual or violent offences.

But 574 others have earned a first strike.

The Sentencing and Parole Reform Act, which established the system of harsher sentences for repeat offenders, came into effect last June.

The system is potentially a hot issue for the election, as Labour's law and order spokesman Clayton Cosgrove has said the law would likely be repealed if Labour won power.

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"It would take something seismic to convince me that three strikes should be retained," Mr Cosgrove said last week, though he did not say what would happen to those who already had strikes to their name.

The policy has been criticised as removing judges' discretion, leading to the potential for sentences disproportionate to the crime. But the Government says prisoners are being deterred from crime, though it has cited only anecdotal evidence for this.

The law applies to one of 40 violent or sexual offences. An offender receives a normal sentence and a warning for strike one, a sentence without parole for strike two, and the maximum sentence for the offence, without parole, for strike three.

Corrections Minister Judith Collins said repeal would be crazy as it would open the door to possible parole for offenders with two or three strikes to their names.

"I would have thought that having people who are recidivist murderers or robbers out early is not something the public would want," Ms Collins said.

"They want to know that the worst recidivist violent and sexual offenders are well secured in prison and serving their time. And if they are up to a second or third strike, then they're quite clearly not getting the message and they need to be kept away from the law-abiding public for as long as possible."

But Rethinking Crime and Punishment director Kim Workman said parole lessened the chances of reoffending, and most "strike" prisoners would still eventually be released.

"There's about a 10 to 15 per cent difference in reoffending rates between those who are paroled out of prison and those who are not."

Figures released to the Herald show that 574 first-strike warnings have been issued in the 14 months since the law came into effect.

More than a third have been for robbery or aggravated robbery, 31 per cent for sexual assault, and 24 per cent for serious assault.

Only one person has two "strikes" to his name; the first for robbery, and the second for aggravated robbery (stabbing), which has a maximum sentence of 14 years.

Convictions have taken place for only nine of the 40 offences, including murder (three times) and attempted murder (twice).

Ms Collins said there were no plans to extend three strikes.

THE NUMBERS
First strikes from June 1, 2010, to July 31, 2011:
* Murder: 3

* Manslaughter: 11

* Attempted murder: 2

* Serious assault (injuring/wounding): 138

* Other acts intended to cause injury: 3

* Sexual assault: 177

* Abduction and kidnapping: 16

* Robbery/aggravated robbery: 198

* Aggravated burglary: 26

Total: 574

Second strikes:
* Robbery/Aggravated robbery: 1