New Zealand's Civil Aviation Authority (CAA) regulations include AC43.14 Appendix 9 "Non Aeronautical Equipment" which can be found at
Civil Aviation Authority of New Zealand.
This AC allows for the installation of not only portable but also some permanently installed equipment such as satellite tracking devices without an STC or a 337.
It is incorrect to say that Spidertracks (or, indeed, any other tracking provider) are prohibited from doing what they are doing in New Zealand, in fact the New Zealand aviation community as a whole is well educated on the use of tracking, the tracking vs 406 debate, and the relative merits and shortcomings of these complimentary systems.
Airways Corporation (the NZ government entity responsible for airspace management) is a strong advocate of satellite tracking for the GA community as evidenced by their sponsorship arrangement with both TracPlus Global and Spidertracks (both New Zealand tracking providers).
CAA has adopted a more considered and appropriately cautious approach, waiting for the technology to mature and standards to emerge before making any decision. In light of the recent Transport Canada debate, that approach would seem judicious.