According to Simon Collins at the New Zealand Herald;
New Zealand's murder rate appears to have almost halved in the past 20 years despite an overwhelming public belief that crime has got worse....The murder rate is regarded as one of the best measures of trends in actual violent crime, because it is least likely to be affected by changing police policies or public attitudes which are believed to have affected recent family violence statistics.Collins is basing his observation on police statistics. He continues;
The rate leapt to an average of 21 murders per million people annually from 1985 to 1992, but has dropped steadily ever since.However there are difficulties in using police stats as described in the article.
Based on
conviction statistics using Statistics NZ table builder, from 1985 to 1992 the average annual number of convictions for murder was 27.9, with an average population of 3.375 million, provides a rate of 8.27 per 1 million.
Staying with convictions and the same source, the rate from 2000 - 2007 is 5.93.
Lower but hardly half.
As with car crashes, which I have
argued more people are surviving, more people are probably surviving the trauma of assault. So rather than just murder, let's look at overall violence, which by the way, academics are convinced is an affect of unemployment.
Again, sticking with conviction statistics for violence, assuming the worst offending results in a custodial sentence, you can see that there are much lower numbers over the period of high unemployment with a steady rise from 2002.

And just for confirmation, let's look at Police statistics. Importantly these show the recorded
rates of violent crime climbing from around 68 per 10,000 of population in 1988 to 140.4 in 2008.

So in summary, for whatever reason, the murder rate appears down. It isn't because there is less violence.