Leaving aside the arguments regarding fairness, retail level playing fields, etc etc, how on earth would/could such a proposal work?
I really cannot believe that my Chinese or Taiwanese supplier of various dribs and drabs of computer gear (for example) will go to any effort whatsoever to register with the NZ Government or provide 'appropriate' specific documentation for NZ GST collection.
More unlikely is that they would subsequently report and remit funds so collected to NZ.
Also, with a purported $60,000 per year trading limit, I foresee a heap of 'trading names' ensuing (as they already do now on eBay etc), that will just be entities of a larger organisation that will come and go as the magical NZ$60,000 figure is approached.
Stupid, stupid, stupid !!