The commercial nature of Tomtom is not what is important. Garmin is commercial and has plenty of free and open source map support. Unfortunately there isn't so much support for the Tomtom. You could do a search for Tomtom on the NZ Open GPS Project forums http://gwprojects.org/forum/
That may be true for garmin but Tomtom choose to sell their maps and I doubt they would allow others to give map software for their GPS's away free when you must pay for the geniune copy.
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