I have found a way to get offline GPS topographical mapping on my phone (i.e. you can use it without being in a cellphone coverage area).  This is useful for tramping, mountain biking, off road running, etc.  It is not as good as using a real NZ topo map but it isn't too bad.

The source of the topo maps is http://www.openstreetmap.org/
Select the "Cycle Map" layer using the blue (+) button on the right hand side of the map.
The OpenCycleMap tiles seem to be a little bit hit and miss, but the level of detail is good.

The mapping application I have been using is Run.GPS.  It is a paid application costing NZ$35.  However, I am pretty sure you can create offline maps for a few other applications, some of which are free.  See http://trekbuddyatlasc.sourceforge.net/index.html

To download maps for offline use in Run.GPS, go to Configuration/Map Settings/Items on Map.  Select the OpenCycleMap layer, and select "Download Now" to download the map shown in the navigation screen (it will download all zoom levels).

By the way, Run.GPS is pretty cool.  You can do a 3D google earth fly through of your routes, while monitoring your pace, heart rate and all that guff.  I am sure it would be useful if I was some sort of athlete (ha ha)