have done some research and this is my semi-conclusion.
pre loaded cards - best option is kiwibank probably for ease of use/setup but the exchange rate for this is worse than exchanging for cash.
the rate for cash or preloaded cards is both not as good as the rates by visa/mastercard if you just swipe your credit card (even taking into account the % fee charged by your banks)
the % fee charged by the bank varies quite a bit from 1.85% at kiwibank, 2.1% at asb and 2.5% at bnz (or something like that) and also depends if whether its a credit card or a visa debit card.
I'll probably take a small amount of exchanged cash and use my googlepay/credit card for anything else.
locking the cards from phone app is a good idea but if the phone is lost/stolen then you're pretty much screwed cuz you cant get the app activated without your sim etc even if you get a new phone or use a friends one. lol