NZ cities are neither warm nor humid. Generally speaking, NZers don't heat buildings the way Europeans do. We are too tough.
There's really no way any of the importable species could survive in the wild in NZ, even in the far north. But yes it is a bit silly that tarantulas (and the two species of scorpion which are also on the allowed list!!) can be brought into the country for zoos but not a single male non-venomous tropical snake can be. But that's the way it is.
If anything goes wrong, we shouldn't take that as a kismet, should we? (Specially when we are as tough as Newzealandians
I like to mention, that this is not a NZ bashing, it is just against some - hmh, lets say: weird - decisions of authorities (as we have had recently in Switzerland by stopping the import of dolphins for example. And we have just one (1!) dolphinarium/dolphin lagoon in an amusement park).