Yep, it works completely "off-line". If it is outsie the 3G/4G network it cannot chew up any data. You download the maps which can be 1 - 2 Gb to the device's SD card before you leave.
We used it all the way across the Great Central Road, Savanah Way, up the OTT with no problems showing the maps, current position, recording routes and so on.

