To be really honest, whilst a GPS system is very useful as far as I can work out, coz I don't own one. I don't think that New Zealand is a country that you really need one.
Sure, one can always find an excuse to use anything, but the place is so small and so empty that if you firstly know your destination, two, know the general direction, then using the sun as a guide you can work out which road/track to take.
The best part is that if you do get lost, don't worry, all the roads are great and you're sure to be having a fantastic ride.
Mick.
Ps:- if the road/track starts getting vertical you know you are about to cross a mountain range, when it starts to go down rapidly, you know you've crossed the range!