"Evidence" touted by National Party leader Simon Bridges as proof the Three Strikes law works is not actually the proof it is claimed to be, according to the Ministry of Justice.

The "evidence" appears to show a drop in the number of people appearing before courts for "second strike" offences - but it fails to take into account five years' worth of change in criminal justice.

Minister of Justice Andrew Little is going to Cabinet today with a plan to get rid of the law, saying "tough on crime" policies are not working and have been shown to have made New Zealand less safe.

The claim found support from the Prime Minister's Chief Science Adviser Sir Peter Gluckman in a recent report which said "tough on crime" policies leading to longer time in prison created inmates who posed a greater risk to the society into which they were eventually released.

In an interview with the NZ Herald on Three Strikes, Bridges cited information released under the Official Information Act (OIA) as evidence of the value of the law.

The OIA data from the Ministry of Justice shows there had been a drop in the number of people convicted on "second strike" offences since the 2010 law was introduced.

National leader Simon Bridges is planning to bring "three strikes" back if he takes his party into Government. (Photo / Peter de Graaf)

Bridges said: "The OIA material … shows that apples with apples, in comparison with the same sort offences and trajectories in the past, it is reducing people graduating on with those second and third strikes.

"They get the warning, they heed the lesson that they hear and they are very reluctant to move on to the next strike."

Told the Ministry of Justice had applied caveats to the information, Bridges said: "Let's even say that's somewhat right … at the very least it certainly doesn't show it's not working."

Bridges said the law was applied rarely and wasn't placing a strain on prison numbers. "I'd say to Andrew Little, why are you getting rid of it if you aren't trying to burnish your liberal credentials?"

Caveats on 'proof'

Ministry of Justice insights manager Anton Youngman said "important caveats" had been included in OIA responses which showed the statistics were not evidence of Three Strikes working.

"Essentially we were pointing out to requesters some of the important caveats that needed to be considered when interpreting the data, and the difficulty in comparing the period before the act came into force, with the period after because of the many changes that occurred."

He said there were changes to policing and prosecution practices, and a "significant reduction" in the number of cases before the courts.

"In other words, there is no way to pinpoint behavioural changes … to the law change alone and attempting to do a like-for-like comparison when there are so many differences is very challenging."

The statistics show about seven people each year were picking up "second strike" warnings and being forced, under the law, to serve the full term of the sentence they received without being eligible for parole - even if mental health and rehabilitation programmes had been completed.

Under the law, a "third-strike" offence - being convicted a third time on one of the 38 crimes on the list - resulted in a maximum possible sentence without parole.

It was this part of the law which resulted in two convictions of indecent assault - one for an inmate pinching a prison officer's bottom and one for a man with sexual convictions kissing strangers - being sent to prison for seven years.

The law has also seen some second-strike prisoners being given lower sentences after defence lawyers argued that the "second strike" forced prolonged periods in prison which were unfair when compared with similar sentences.

It also fails to include some crimes of violence which carry sentences beyond a maximum seven years in jail, including throwing acid on someone, killing of an unborn child or treason, which includes attacks on the Queen or starting war against New Zealand.

Ill-treatment of children, slave trading and piracy involving murder are also not on the list of "strike" offences.

'Ad hoc' crimes with public appeal

University of Otago law professor Andrew Geddis said the law was an "ad hoc" collection of crimes that sent a message that had "some success with the voting public".

"I suspect they had in mind a kind of stereotyped offender [and thought] 'what might that offender do to people' and put these offences into law based around 'who are the sort of people we want to target'."

Geddis said the law was inconsistent with the principles of justice as practised in our courts.

"The major disconnect is that the three strikes regime requires judges to impose predetermined sentences on people irrespective of their actions or overall culpability."

Act leader David Seymour said the data showed an "admittedly statistically weak correlation" between strike warnings and reduced offending.

But he said the law meant criminal justice had shifted towards the public view and there was "nothing illegitimate about that".

He rejected Gluckman's finding that "tough on crime" pandered to politicians playing on public concern for votes.

"I've got a lot of time for Peter Gluckman but he's exhibiting signs of intellectual snobbery.

"On what basis is his experience and evidence worth more than someone who lost a loved one to crime or had their shop done over?"

Former Act MP and lawyer David Garrett, who wrote the draft law, said the initial list of convictions was "random" but had been expanded to include all offences of serious violence which carried a maximum penalty of at least seven years.

Acid throwing was deleted because no one could remember the last time it had happened, he said.

Garrett said he was seeking new data going back earlier to measure the impact of the law, which he believed had a deterrent effect. He said Ministry of Justice officials at the time were "heavily biased" against the proposed law and provided figures which could not be relied on.