The Department of Corrections says its cellphone jamming programme is working - in spite of a loophole that allowed an alleged drug ring to be run from a Christchurch prison.
The inmate at the centre of the allegations was caught in a police operation in February.
The department's general manager of systems and infrastructure, Derek Lyons,
says he understands the inmate was using an advanced 3G mobile phone.
Mr Lyons says the jamming technology installed at Christchurch Men's Prison late last year is still being tested to detect 3G phones, but it is just a matter of time before it can.
The $5.7 million jamming programme has been installed at 16 prisons since 2007 and is being rolled out at four remaining prisons.
Cities have been left until last because of the potential for interference with mobile phone networks.
Mr Lyons says Corrections will work with telecommunications companies to tune its jamming programme as new cellphone technology is brought out.
The Telecommunications Carriers Forum says it will monitor cellphone blocking technology in prisons, to make sure it does not interfere with mobile networks.