MapApp is a universal iPad/iPhone app which has all LINZ 1:50000 Topo50 maps built-in for offline viewing.
There are separate apps for the North and South Islands because its a bit of squeeze getting all that data under the 2Gb maximum app size.
The apps are pretty simple – GPS location and placename search are supported but no gpx import/export yet.
If you travel in areas with no cellphone coverage then this is the app for you!
MapApp NZ North Island and South Island are $19.99 each on the App Store.