I believe Dishtv (http://www.dishtv.co.nz) is the NZ supplier. They used to have them listed on their website but don't appear to be there any more. Magic tv website (http://www.magictv.co.nz/support.html) has a contact email of firstname.lastname@example.org. Send them an email. There doesn't seem to be any firmware updates in a while so it looks like it's pretty much dead.
I temporarily connected my Magic tv 3600 to a wired network some time ago to see what features were available but it wasn't stunning. From memory it was just some weather/news headlines, but when you looked at the details of the news item, the text appeared to be truncated. I also tried an USB wifi adapter I had and it didn't work either, so yes I believe you need the official one. I suggest you temporarily connect to a wired network and see if it's worth getting. Update this with how you get on.