A respected academic is blaming Maori for high rates of violent crime that have helped give Hamilton the country's second-highest number of "three strike" offenders.

The "three strikes" sentencing law, a key plank in the National Government's law-and-order drive, was introduced in 2010 as part of the Sentencing and Parole Reform Act.

Justice Ministry figures show 275 offenders have received a first warning in Hamilton courtrooms under the regime - second only to Auckland.

Only 54 first warnings have been given in Dunedin.

Canterbury University sociology professor and criminologist Greg Newbold said Hamilton's high number of first warnings reflected the city's large Maori population - a comment one Waikato academic has called an "over simplification".

Prof Newbold said there was a "direct association" between Maori and violent offending.

"The more Maori you have in an area the higher levels of juvenile delinquency and higher levels of violent crime," Prof Newbold said.

"Maori are far more likely to be convicted of violent offences than non-Maori [and] far more likely to be victims of domestic violence than non-Maori.

"There are hardly any Maori in Dunedin and not many in Christchurch," Prof Newbold said.

"The more Maori you get in an area, the more violent crime you get - that's a fact of life."

Waikato University faculty of law senior lecturer Wayne Rumbles said Prof Newbold's comments were "partly right" but also an over-simplification.

"Maori are more likely to be charged with offences, so there's a bit of prosecutorial bias," Mr Rumbles said.

"It's not that if you have got more Maori you get more violent crime necessarily, but you get higher charging of violent crime by the police."

Critics of the three strikes law say the regime has little to no effect in deterring crime and can result in sentences disproportionate to the third offence.

A "third strike" conviction attracts the maximum penalty for that crime with no parole, unless the judge rules it would be manifestly unjust. Forty offences are covered by the three strikes law.

Prof Newbold said he opposed the three strikes legislation because it took away a judge's discretion. "It [three strikes law] is a Government vote of no confidence in the judiciary and Parole Board, that's what it is.

"It can't be interpreted any other way," he said.

New Zealand Howard League for Penal Reform chief executive officer Mike Williams said the three strikes law was unproven at deterring crime and an "extremely expensive" crime fighting measure.

It was a drain on taxpayers to lock offenders up for maximum terms, Mr Williams said.

It was better to spend money on prisoners' rehabilitation, and drug and alcohol treatments.

Corrections Department figures obtained by the Waikato Times show the department spent $757.2 million in 2012/13 keeping 8625 criminals locked up. More than $13.4m was spent on 83 maximum security prisoners in 2011/12, compared with $4.03m in 2006/07.

Corrections Services deputy national commissioner Maria McDonald said the management of maximum security prisoners was constantly evolving to better assist prisoners toward a crime-free future "if and when they are released".

Hamilton barrister Roger Laybourn said the general view in the defence bar was that the three strikes law was "another ill-researched bright idea".

He said there was no evidence the regime would reduce crime but could cause problems when a repeat offender appeared in court on a minor but qualifying offence.