New Zealand's family violence problem is only getting worse yet police are prosecuting fewer people over the crimes in a trend described as "disturbing" by support workers.

Figures released to Mike Hosking Breakfast under the Official Information Act show police launched 121,739 thousand family violence investigations last year - or 333 a day. 

That's up fro 73,280 in 2008.

The number of offences is also increasing – totalling nearly 39,662 in 2017. 

Yet as those numbers increase, the number of apprehensions and prosecutions is trending down with 16,764 prosecutions made last year – down more than 2500 from 2008. 

Women's Refuge chief executive Dr Ang Jury says she would need an explanation from police as to why offences are up yet apprehensions and prosecutions are down, describing it as not only surprising but disturbing.

"If there is an offence identified as there appears to be in 16,000 cases last year, yet only 8000 of those result in an apprehension - what is that about?" she said.

"How do they explain that failure to apprehend - they know who the victims and perpetrators are when they attend."

Police suggest the drop in prosecutions is due to its officers looking for alternatives to arresting and charging perpetrators and say they have put extra resources into tackling family violence.

As part of this, police were leading two pilot programmes in partnership with other Government agencies, that took a "whole of family-whanau approach", national prevention manager Superintendent Eric Tibbott said.

The programmes aim to ensure the immediate safety of victims and children, while also working with perpetrators to prevent further violence.

"The focus is to engage with the family, work through the issues, then actively support them to navigate their way to wellbeing with a co-ordinated response to their goals and needs," Tibbott said.

He said family violence statistics were up because police now investigated every call they received related to family violence.

But he thought another reason why the statistics had increased was because victims were gaining more belief they could get help from police.

Jane Drumm, the general manager of domestic abuse support group Shine, agreed police had put more frontline and follow-up officers towards tackling family violence.

"They realised it is an underlying cause of a huge amount of all sorts of crime" and that it takes up on average across the country about 40 per cent of the time of frontline staff, she said.

"So strategically it is a really important issue for them to try to get on top of."

But she said community groups had not been given more funding at the same time so they could step in and work with families and perpetrators who might no longer be prosecuted.

"In the last Budget we did get some extra money but we've got a long way to go before we can really respond in the way that New Zealanders want us to," she said.

University of Auckland professor Janet Fanslow said she was not "overjoyed" to see police and the courts prosecuting fewer people for family violence crimes.

One study had estimated that only 13 per cent of all family violence was ever reported to police, she said.

So if police were being more successful at getting crimes reported, then you would likely expect more prosecutions, she said.

She also said the main way those committing family violence started counselling or other help programmes was by order of a court or police process.

International evidence showed they were then more likely to continue with the programme if they were being monitored by a court or similar power.

"So if we are losing that avenue because we have less prosecutions, and we haven't really funded other avenues for them to try to change their behaviour, then we've got a problem," she said.