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If you do any boating MX Mariner is like $10 bucks and has charts for all of NZ freely downloadable, zoomable, and integrates with your tablet or phone GPS and can do tracking and waypoints and measure distances and so on. With this you pretty much don't need a chartplotter, and way cheaper than Navionics if you're only in NZ waters.
MX Mariner ($9.99) is a great choice for countries where maps are available. The US coastal maps are made free by the NOAA.
edit: added price