Constable Jesse Singh with an I-Phone 6 plus police used to record statements in family violence cases.

A Palmerston North police trial to record domestic violence complaints on smartphones, designed to catch more perpetrators of domestic violence, has hit a snag after the courts found the way the recordings are stored makes them unusable in court.

Justice Minister Amy Adams says she is working to get the law changed to remedy the issue, but one police prosecution is already at risk due to a legal decision senior criminal lawyers expected.

Police announced in December that frontline staff in Palmerston North would be armed with iPhones, which they would use to record statements at the scenes of alleged domestic violence crimes.

FAIRFAX NZ Justice Minister Amy Adams says she will try get regulations changed to ensure iPhone recordings can be played in court.

While complainants usually have to give their account of events from the witness box during trials, the idea was to have the videos played in court as "evidence in chief", with complainants then asked additional questions.

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Police believed it would prevent victims recanting on statements made at the scene to first responder officers.

But that is a situation that may play out in one case, after Judge David Smith ruled an iPhone video could not be played in court because the way it was stored was at odds with the Evidence Act and its regulations.

Police applied to have the iPhone video of one complainant used as evidence in chief at a judge-alone trial for a domestic violence case, with court documents showing they feared the complainant may not turn up to court or would not give detailed evidence.

The complainant had told police they did not want the video played, and that they wanted the charges against the defendant dropped.

But they judge declined the application to play the video due to the way the information was stored.

In his May decision, the judge said the master copy of video interviews had to be kept in the custody of police.

While that was simple enough with the standard DVD-recorded interviews done at police stations, the iPhone interviews were uploaded from the phone to the same Australian-based servers that hold Taser footage.

The footage is then copied from the servers to DVDs by police staff.

"While the DVD created master copy is kept in the custody of the police, the real master copy, in this case, is the digital copy," the judge said in his ruling.

"That is not in the custody of the police but in the custody of a foreign corporation in a location outside New Zealand."

The evidence was possibly safer in the servers due to encryption technology, but deciding to make that form of storage legal was not up to the court, the judge said.

Lawyer Peter Coles said he and another Palmerston North lawyer raised the storage issue with police when they had a meeting with them before the iPhone trial started.

"We said to them 'you have got an issue' and they did not seem to think they did.

"I have no issue with the technology, but people with bright ideas should make sure they are lawful before they are put into practice."

Video interviews were a great tool, but had to be used in the right situations, he said.

"There are issues that you have to be a little bit cautious of when shoving cameras in the face of people in volatile situations when they are possibly under the influence of alcohol or drugs."

Police national crime prevention manager Superintendent Tusha Penny said police accepted the decision.

Police did not know definitively that a law change would be needed until the judge's decision, she said.

The regulations for interviews "have not kept pace with technological advancements during the past 10 years".

"Our primary focus is victims, and we are always looking at ways to improve their experience with police and the judicial system.

Justice Minister Amy Adams said she was aware of the issue, and would be seeking approval to update regulations as soon as possible so the iPhone trial could continue.