American Territories (Samoa, Palmyra, etc) can be downloaded for free in ENC or Raster ENC formats from
http://chartmaker.ncd.noaa.gov/
In fact the entire US is free. For all other countries you have to pay.
I recommend you do not use the NZ enc's. You can purchase electronic charts in many different formats. If you are using PC based navigation (like Fugawi) then I recommend "softcharts". It will run the free ENC's and a bunch of other formats. Otherwise you'll be forced to use the overpriced proprietary charting of whatever brand of chartplotter you have, Garmin being the worst for the South Pacific.