There's no evidence the much-vaunted Three Strikes Law has made a dent on crime, a law professor says.

More than five years after the law's introduction, Professor Warren Brookbanks said conflicts over interpretation were potentially undermining the new law, and there was anecdotal evidence some judges were forgetting to issue strike warnings.

"Our concern was that it was likely to punish many relatively minor offenders more harshly than they deserved," the Auckland University law Professor said during the inaugural Greg King Memorial Lecture. "I still have this concern."

Mr Brookbanks delivered the lecture at Victoria University of Wellington this week.

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He said the law was created to identify and punish "worst-of-the-worst" offenders but no empirical data so far vindicated hopes the law would deter offending.

Mr Brookbanks said he worried the law was an example of "penal populism" in which politicians served their own ends "by tapping into the public's punitive sentiments".

National worked with the Act party to pass the Sentencing and Parole Reform Act 2010.

Earlier this year, former Act MP David Garrett said the Three Strikes Law was working well, and as intended.

Mr Brookbanks said models abroad inspired the local law, but it was still less harsh than foreign examples such as that in California.

He said 40 offences qualified for the "strikes" including all major violent and sexual offences attracting a maximum penalty of at least 7 years' imprisonment.

He said data released in 2013 showed just three offences - sexual assault, robbery or aggravated robbery, and serious assault - accounted for 89.8 per cent of all first-strike warnings imposed.

Mr Brookbanks said these three crimes accounted for 95 per cent of all second strike warnings during the same period.

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Manslaughter, murder and attempted murder accounted for only 2.9 per cent of first strike warnings over that time.

The professor said by April this year, 5382 first strike warnings had been issued and only 76 final warnings, but this didn't mean the law was deterring would-be recidivists.

"Deterrence, if it is effective at all, should operate across the full criminal justice spectrum, not simply once a first strike conviction has been entered."

He said a narrow band of serious offences were producing the vast majority of strike warnings.

"In fact, there could be an argument for reducing the list of qualifying offences, rather than increasing it, as some have advocated, while developing policy for dealing more effectively with the most prolific strike offences."

These comments were in contrast to those of Tauranga's Sensible Sentencing Trust, which this year called for an expansion of the law to cover domestic violence offences.

The Greg King Memorial Lecture was held in honour of the eponymous lawyer who defended clients including Ewen Macdonald and died in November 2012.