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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.

Police statistics show that for 44 years until about 1970 the murder rate fluctuated around an average of six a year for every million people.

The rate leapt to an average of 21 murders per million people annually from 1985 to 1992, but has dropped steadily ever since.

Last year's rate was 12.1 murders per million people.

Victoria University Institute of Criminology director Michael Rowe said the decline coincided with similar falls in violent crime in Australia, the United States and Britain since the early 1990s.

Unemployment has dropped since then in all three countries, until recently, as have the numbers in the most violence-prone group - males aged 15 to 29, who declined from 12.3 per cent of New Zealand's population in 1991 to 10 per cent in 2006.

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.

Dr Rowe cautioned that even the reported murder rate over a very long period could be affected by changes such as the numbers of police officers and police stations.

"Urbanisation may make it more likely that murders occur, or that the murders are recorded," he said.

"Obviously murder is the classic crime where it's estimated that very high proportions of [actual] murders are reported to police.

"But it's possible that has changed over a long period and that people are more likely to report things as murder than perhaps they were 50 or 60 years ago."

Police statistics manager Gavin Knight said the introduction of the Wanganui computer in the mid-1970s led to a change from recording "cases" to recording "offences", with multiple offences often counted for each case.

He said the definition of murder might have changed, too.

For example, conspiracy to murder, attempting to procure a murder and accessory after the fact to murder had all been included in the offence type "murder" since 1994.

"How such offences were treated in earlier years is uncertain," he said.

However, even if these offences were not counted as murder before 1994, the effect of excluding them may be relatively minor.

They account for only 7 per cent of all offences classified as murder since 1994.

All these statistical changes would also appear to have increased reported murder rates, and seem unlikely to account for the dramatic fall in reported rates since 1992.

Despite this decline, Dr Rowe said a public perception that crime was getting worse remained.

An institute survey of 1400 people in four parts of New Zealand - including South Auckland - found that 80 per cent agreed or strongly agreed that the country's crime rate was rising. Only 4 per cent disagreed.

Yet the same survey - which has yet to be published - found that only a quarter of the people surveyed believed crime was rising in their own neighbourhoods.

When asked where they got their information about the national crime rate, people said from the media.

* The figures

Annual murder rate (per million people)
1926-1970: 6
1985-1992: 21
1994-1998: 15
1999-2003: 14
2004-2008: 12