It's quite easy to use Kindle Unlimited in NZ.  Under "Your Account", choose  "Manage Your Content and Devices".  Go to the "Settings" tab and look in "Country Settings".  Click the change button and choose a US address.  Now you can go to and get a 30 day trial.  You can use a NZ credit card without a problem.

I've read 4 books so far, have 10 on my phone and another 40 in my "Unlimited" wish list.  This should keep me going for a while.