I took it to the Samsung repair centre who refused to do anything as, although it had been bought in NZ, it wasn't a model that Samsung officially sells in NZ. The warranty wording is "This Limited Warranty is valid and enforceable in the country where you have purchased the Product, provided that (i) SAMSUNG has intended the Product for resale in that particular country and (ii) SAMSUNG Warranty Service is available in that particular country."
The supplier took it back for repair (2 weeks) but the same problem occurred when it was returned to me. They have now refunded me. Two months wasted with repeated factory resets, safemode tests, ...
It seems that although Samsung is a global supplier they don't provide a warranty that is suitable for people who work / travel in multiple countries and might need service in a country other than where they purchased their device.
If you wish to purchase a Samsung, then buy the NZ model G920I.
Beware of parallel importers.