The Department of Corrections is playing down concerns "foiling" could impact public safety but police say it's "not good" that offenders are finding ways to give authorities the slip.
It comes after the New Zealand Herald reported the technique of "foiling", which is when criminals wrap tinfoil around their ankle bracelets to interfere with the signal, is happening regularly, according to a leaked internal police report.
Corrections told AM while instances of tampering are very low, the technique of using foil makes it easier to bypass security measures.
Corrections National Commissioner Leigh Marsh told AM on Monday he didn't have stats for foiling, but 1.4 percent of offenders wearing ankle bracelets tamperered with them in July.
He said there are around 6300-6500 people being electronically monitored across all sentences and orders. So 1.4 percent of 6500 people equals 91 people who tampered with their ankle bracelets in July.
Marsh said while most people do comply, he admitted the technology can be "challenged".
"The tampering can range from anything from cutting the bracelet off where the fibre optic cable within the strap is cut and that sends an immediate alert through either the cell networks or the base units or trying to shield the device or other methods people may take to evade monitoring," he said.
He told AM co-host Ryan Bridge foiling doesn't necessarily mean Corrections don't know where criminals are, it just means they have to do a little bit more work to understand the type of signal.
"If the device stopped communicating with the home unit, normally, we would look to see whether they've moved into a part of a building where the coverage is slightly less than it would be," he said.
"It's like when you walk around your house with your cell phone or your Apple Watch and you sort of lose signal as you move through denser parts of the building. But if we start to lose that signal in the building and it's picked up outside the building, that's when we know they've left the premises."
He told AM it's very hard to stop cell signal, but couldn't say for certain if a signal would continue to be received when offenders wrap tinfoil around their bracelet.
The police report outlined specific examples of people putting tinfoil around their ankle bracelets to commit crimes.
One detailed a man who foiled his tracker and went to his ex-partner's house where he allegedly tried to stab her before fleeing the scene and cutting off his ankle bracelet. The EM (Electronic Monitoring) bail team was unaware the man had left his bail address until police contacted them. The man was on the run for three-and-a-half weeks until he was found.
Other examples provided in the report were of youths as young as 14 committing ram raids and smash-and-grab burglaries.
When asked about the example of the man who allegedly attempted to stab his ex-partner before fleeing, Marsh played down any safety concerns to the public.
"From our perspective in regards to the numbers, the majority of people do comply but for those that don't, and when we detect that, we report that and respond to it as quickly as possible."
Police Assistant Commissioner Naila Hassan told AM while they're aware of the foiling, they don't know how widespread the issue is.
"What I do know is that some offenders are making their way around the process by foiling their bracelets and they are committing offending. Our role as police is to make sure that when we are notified of breaches we respond accordingly," she said.
When asked if she was disappointed by Marsh's response, Hassan said most people do comply with their bail conditions but did say when offenders don't it's not a good look.
"It's not good that offenders are finding a way to get around the bracelet and offend, absolutely."
Watch the full interview with Leigh Marsh and Naila Hassan in the video above.
