I use back country navigator and find it pretty good. I've only used it for topo + gps location so far though, haven't tried it for navigating to waypoints etc.
Not sure why you'd bother with custom maps etc when apps like back country navigator (and presumably nztopomap) use LINZ topo50 anyway.
As ebf said the biggest issue is battery life, and remembering to download the map area in advance.
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