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"?
Neither am I a lawyer, but the key part is if you're (a) doing this as a regular security precaution or (b) doing it specifically to evade customs. If it's the latter, it's probably illegal. If it's being done for legal reasons, it's legal.
Something I've noticed those of us who work with computers having difficult with when it comes to the law is the way intent changes the legality of an action. Wiring money to person X is generally legal, unless done so for the purpose of acquiring illegal contraband. At that point, both the illegal acquisition and the wire become illegal.
Neither am I a lawyer, but the key part is if you're (a) doing this as a regular security precaution or (b) doing it specifically to evade customs. If it's the latter, it's probably illegal. If it's being done for legal reasons, it's legal.
Twice now you've expressed this belief. Please share some primary sources that inform your beliefs.
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"?