The communications regulator says it recently destroyed 100 illegal jamming devices. The ACMA's field staff can also seize jammers by tracking them using special equipment, but most are found in the international mail stream before they can be used by their purchaser. According to the ACMA, jammers are typically used by those who want to stop mobile phone calls from being made or received in a certain vicinity. Others just use them to cause a nuisance. In one example several years ago, the regulator found an imam at a Western Sydney mosque using one during prayers to ensure there was silence. In another, a company installed one in its boardroom after getting advice from a security expert. Previous figures revealed by Fairfax Media in March last year showed for the first time the extent to which mobile phone jammers had been seized from international mail packages. According to the latest figures, it appears less jammers are being seized by authorities. Only 15 were confiscated in the first half of 2013, compared to 89 in the same period last year and 141 in the first half of 2011. The decrease in numbers could mean less are being imported or that importers are avoiding detection, according to Mark Loney, executive manager, ACMA's operations and services branch.

Jamming devices can interfere with wireless networks and cause disruption. "My sense is that the rate of intercepts to the mail stream is falling," he said. "That could be a good sign and that there's less people buying them. Or it could mean that they are coming in and are not being picked up in the mail stream. We don't have perfect knowledge about this but the fact that we're seeing less is encouraging." The ACMA destroyed the jamming devices after a crackdown. Mr Loney said international mail packages were inspected by a sophisticated x-ray machine that can automatically detect devices by shape and then alert mail-sorting staff to take a closer look if necessary.

"With the jammers some of them have quite distinctive shapes," he said. "They have tri-band antennas that stick up, which you don't see on a normal mobile or any sort of small electronic device. And when [mail staff] see something like [a jammer] it gets pulled off and they look at it." Despite this, Mr Loney admits that some can still slip into the country. This is where the ACMA works with telcos including Telstra, Optus and Vodafone which sometimes receive reports from customers about mobile phone interference. ACMA's staff then go out and attempt to track suspected jammers with specialised radio equipment. Of the 211 jammers confiscated since July 2011, 190 were found in the mail stream, 19 were seized by police and two by ACMA's field staff. Jammers can, however, be operated if an exemption is granted by the ACMA. The NSW Department of Corrective Services is the only approved organisation in Australia to be granted an exemption. In February this year it tendered for the installation of a mobile jamming system for use at Lithgow jail, about 141 kilometres west of Sydney, as part of a trial.

If the trial is successful, the department hopes to roll out jammers to other prisons in NSW. The idea for a jammer trial at Lithgow jail was first floated after a prisoner was found to have smuggled a mobile phone into their cell in 2008 to run a drug ring operation on the outside. This reporter is on Facebook: /bengrubb