Didn't have to download anythnig other than the voice add-on app, which it prompted.
The pre-loaded app is called 'Navigation'. It automaically detects where you are, but you need to be connected (wifi) to provide a destination address. It then plots a route. Then you can drive off (outside Wifi range) and it seems to already have what it needs to get you there. Not sure at what point it downloads all the maps. Even without providing a destination it keeps updating maps as you drive along, so clearly the provision of a destination (when within Wifi range) isn't the trigger. Must just be magic!
Might help that I'm driving in central(ish) Auckland (going from Upper Queen Street to Mt Albert and back)? Haven't been anywhere else to see how it fares.