Just to clarify, the 3G iPad is a true GPS, and doesn't use phone networks to locate it's position. The VMS app uses mud maps, and includes a heap of Itopo off road maps plus Gregorys maps covering all Oz. You can buy more maps on mud maps.
It will work anywhere a "normal" GPS works.
Internet connectivity is a separate issue. I got 12 months Telstra Internet prepaid for $150 from memory. Then you can use it as a simple laptop and surf the net anywhere there's Telstra mobile coverage.
Cheers,
Cam