My knowledge of driving in NZ is limited to watching the Brokenwood mysteries,
and my Knowledge of using google maps on the move is not much better. But I believe you can download google maps onto your smartphone to use off line in areas without a phone signal (or in my case too mean to pay for mobile data) It does require some preplanning prior to the trip,. I did try once to download a largeish area of Europe in this way but it wasnt really practical due to overlap issues but might be possible with some perseverance. But I believe if you have a trip calculated it will download the necessary maps for that route, but not far beyond it either side if you divert off the route.
And without Data you wont get any traffic updates etc. But how common are these in areas without a mobile signal. " Did Kiwi on the road 10km ahead" ( Sorry guys no offence intended)
But it must be really annoying having to find work rounds for stuff that should already be there.