Are you sure it's from the Army mate? And from 2004? Usually they only keep a "white fleet" vehicle for about 3-4 years, and swap it out. Their cars are supplied through lease plan and I've never seen a Nissan supplied. Used to be Holden but now ford. Landcryisers but now Prados and klugers. Patrol utes but now triton tray backs. Taragos but now iMax. Never a Navara.
But hey, I didn't pay attention to the entire army while I was there, so I suppose it's possible. Just sounds strange.
Enjoy it though mate. Is it a DX model? If so, then that's more probable than if it were a STR, as we were only given base models too.