My sense is that the rate of intercepts to the mail stream is falling. That could be a good sign and that there's less people buying a portable cell phone jammer. 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.

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. 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 see something like it gets pulled off and they look at it.

Despite this, government 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 GPS blocker jammers confiscated since July 2011, 190 were found in the mail stream, 19 were seized by police and two by ACMA's field staff.