I'm not so sure it's that clear. (I'm not a lawyer)

If you can lawfully download a phone image over the internet while in the country and you can lawfully enter the country with no or limited digital information on your person then I don't see how backing up your phone in a different country (where New Zealand has no jurisdiction) and legally downloading the backup in New Zealand is evading customs.

Another (presumably) legal scenarios would include backing up all of your data, carrying no phone into the country, and then purchasing one while there. Is that "evading customs"?