Footage showing high-security prisoners flaring up at guards has been released halfway through a trial of body cameras.
They seem to be working – assaults on guards have dropped 70 percent. Police are watching with interest.
The video from the cameras provide a rare look behind the wire. Prison guards race out to a fight and hit record on the body camera.
The prisoner's not only resisting, he's making threats. The body camera means they have got evidence.
"We've put them in place to improve staff safety and we've seen a 70 percent improvement in safety," says Corrections chief executive Ray Smith. "That's 70 percent of incidents where staff have been assaulted."
For the past three months the devices have been trialled in high-security parts of Rimutaka and Auckland Prisons. Eventually Mr Smith wants them in all high-security units across the country.
Similar cameras are already being used by police in the UK. Police here are watching the prison trial with interest.
"For the purpose of collecting evidence or officer safety, it's certainly something we're looking at," says Police Commissioner Mike Bush.
Assaults on guards frequently stem from prisoners not wanting to hang up the phone. But one prisoner caught on Corrections' body cameras eventually complies, with a telling message on the way out – "You're f**king lucky you're wearing that camera."
But while the cameras provide evidence and protection, there's scope for abuse too.
"The concern is that when an inmate's being assaulted, there could be a period of extreme provocation that's not being recorded," says lawyer Michael Bott.
"It's up to the officers concerned," says Mr Smith. "I trust them."
The cameras are each worth about $1400, but Corrections says they're already proving invaluable.