On expats: I can only speak from my own experience, and of those that I've known personally - both here and back in Oz.
My experience with old "Continentals" like myself is that we have moved countries "for good" in our heads a long time ago, and thus we support our local teams here, because this place is our home. Otherwise you are just stuck in the long-gone past. Ironically, and somewhat unexpectedly, this process is also to do with having to adopt a new language, in my case English. This (new language) draws a very sharp line between "back over there" and "here now". It requires you to make a conscious decision as to who you are, where you live, and who you identify with.
In contrast, relatively familiar cultural surroundings, language, and English heritage still strong in NZ may put less pressure on an average expat Brit here to even pose that kind of question .As a result, one could say that they physically move here, but maybe not quite as much in their heads. This could then contribute to a lesser acceptance of what they find in the Antipodes, and then leads to the infamous jibe of a "whingeing Pom".
Add to that the relatively full-on saturation of the local football coverage with all matters EPL at the expense of say the HAL, and it becomes easier for the expat to ignore the inferior product (A-League) in favour of something better.
Ironically, the same does not apply to small local football clubs here,where many UK expats participate in both playing and managing, enthusiastically and effectively. Clubs here in ChCh thrive thanks to their daily grind. Such hands-on involvement promotes loyalty and a long-term club / community commitment, but that is probably harder for a club like the Nix. As there is only one Phoenix, for those UK expats who do not live in Welly the hands-on involvement / support / is harder and probably limited to TV viewing and to writing here, and so Phoenix probably loses out to EPL in head-to-head competition for attention.
These are only my thoughts and I am sure that experience of others with this will be different - I will be happy to hear your views.