"I find it extraordinary," she said.
The decline coincided with a new power given to police from July 2010 to issue "police safety orders", which require an alleged offender to leave the family home for a few days, she said. The orders were supposed to fill a gap in cases where there was no evidence of an offence, but Ms Hannifin said they appeared to be linked to fewer offences being recorded.
"The question to be asked is whether or not [recorded] offences are so low because police safety orders are being used inappropriately."
Jane Drumm of the Auckland agency Shine said prosecutions had also dropped because of new prosecution guidelines issued by the Crown Law Office in 2010, and updated in 2013, which "raised the bar" of evidence required for prosecutions.
The new guidelines encouraged prosecutors to make "plea arrangements" with defence lawyers where "releasing the saved costs in court and judicial time, prosecution costs and legal aid resources [could] be better deployed in other areas".
Ms Hannifin said: "We are dealing with a police force that has been told to lower the [reported] crime rate, we have a court system that has been told to speed up and save costs, and we have an issue that isn't spoken about very much."
However, Police Acting Assistant Commissioner Dave Trappitt said police had always followed Crown Law's prosecution guidelines so the recent changes "did not impact on the decision-making within the police prosecution service".
"Each charge requires the test for evidential sufficiency to be passed before then applying the public interest test," he said. "This has not changed."
A Crown Law spokeswoman said: "There was no intention in these changes to raise the bar for domestic violence cases."
On the web:
www.nzfvc.org.nz
Fluctuating figures
• Domestic violence agencies concerned about drop in family violence prosecutions over past four years.
• Prosecutions for male assaults against female, applications for protection orders and prosecutions for breaches of protection orders have fallen by between 14 per cent and 29 per cent.
• The number of police investigations into family violence incidents climbed from 86,800 in 2010 to 95,100 incidents last year.
• But investigations that led to an offence being recorded dropped from 45,500 to 37,900.