I'm not entirely sure about this- there's fairly heavy truck and delivery traffic throughout the North Island, making it much more likely to spread the virus from Auckland to Wello than it is, say, from Auckland to Christchurch (and we had truckies as a culprit several times both over here and in Australia).
Saying that- in general, it's my personal opinion that it's now time for NZ as a whole to stop this nonsense and follow the rest of the world (sans Tasmania, WA, Queensland and North Korea) to open up, period. We tried the zero case policy with Delta, it didn't work (though I still think 2 more weeks at level 4 would have potentially done it- why lock Auckland in for weeks and then give up right at the moment where it was almost under control again?) as it didn't work over in AU either.