Very likely the Aussie police won't say exactly. All they have said so far is the devices are modified smart phones and they compromised the devices with a supply chain attack.
The handsets, which were a modified smart phone, were sold for about $2350, which included a six-month subscription to an encrypted network and tech support.
The administrator regularly pushed out software updates, just like the ones needed for normal mobile phones.
But the AFP was able to modify those updates, which basically infected the devices, enabling the AFP to access the content on devices in Australia.