Whanganui has hit historically low prosecution figures and a senior lawyer says police budget constraints are part of the reason.

Statistics New Zealand figures reveal the number of adults prosecuted and convicted in Whanganui courts last year was the lowest since at least 1980.

Both numbers had fallen every year since 2009, and had dropped by more than 33 per cent each.

Lawyer Stephen Ross said the downward trend was because of police cost pressures.

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"It is a budget issue. I don't have any doubt about any of those directions. They're related to resourcing. There's no doubt about that," said Mr Ross.

In 2009 there were 1746 prosecutions in Whanganui; last year that figure was down to 1161. Likewise, the number of convictions seven years ago sat at 1539, dropping to 1019 in 2015.

In a written response to Mr Ross' comments, a police spokesman said the baseline budget had not been frozen and its operating funding would increase $41 million per annum over the next four years.

The spokesman said police were making operational changes around the country "to serve their communities better", but said there had been no reduction in staffing.

While the prosecution numbers were low, the percentage of convictions was at one of the highest levels it has been in the last 35 years, with 87 per cent of people prosecuted in court last year convicted.

Mr Ross said a strong conviction rate was intertwined with the drop in prosecutions.

"The police are not pursuing cases where the evidence is not as strong as they'd like," he said.

"If it's not a strong case then they're probably exercising their discretion not to pursue it."

Both the New Zealand Law Society and Criminal Bar Association have expressed concerns at police and Crown Solicitors initially laying "inappropriately serious charges" against offenders. The police acknowledged this occurred but said it was usually due to inexperience.

Mr Ross said laying the most serious possible charges allowed police to offer concessions on lesser charges if the defendant pleaded guilty.

"The police lay serious charges knowing that there's always room to manoeuvre, and that the client won't usually want to risk being convicted on a more serious charge," he said.

"That happens all the time."

Police association president Greg O'Connor said higher conviction percentages were influenced by the increased use of warnings for low-level offences.

However, he said prosecuting fewer people wasn't necessarily the best policy.

"Police have way overshot," Mr O'Connor said. "I don't think anyone believes the behaviour on the street has improved at all. I don't think anybody believes there are fewer people offending or that there's less lower-level offending taking place."

In 2011, the Ministry of Justice announced a target of reducing crime 15 per cent by 2017. Since then, nationwide prosecution and conviction numbers have dropped 28 per cent and 27 per cent respectively.

Mr O'Connor said offenders were realising low-level crimes would often go unpunished and might result in a future crime boom.

"Now you have a generation of people who have grown up understanding that actually there are no real consequences for a lot of behaviours, other than the inconvenience of being dragged back to a police station and kicked out an hour later.

"They have no fear of the system."

Mr O'Connor said he was unsure how the long-term effects of pre-charge warnings would balance out against the short-term benefits.

"If you are the government and want to say, 'the number of arrests have gone down and the number of court appearances have gone down', then yes, it's a great policy, whether it's done with a long-term understanding about the consequences and perverse incentives you might be introducing into the system."