hey headfinder! i messed around with a few things, i hope that's okay. this is still a rough draft, and i'm not SUPER up on my NZ/AUS geography, but i do know a couple of tough facts when it comes to a surviving ANZ area.
the tough part: much of the two countries, even the southern fringes, are simply too warm to eradicate trolls, even in the winter. tasmania gets hit especially hard by this: with few high mountain ranges, most of the island just doesn't get cold enough to sustain reliable snow. the one possible exception, from a little research, is queenstown (AUS) and the mountain it sits at the foot of, which are high up enough to get a couple days of snow a year. it's likely that people would move higher up mount owen and mount lyell and reinhabit a few of the ghost towns from the abandoned copper mine. however, with a habitable area this small, and hemmed in by the rest of the country, which would serve as a festering troll breeding ground in winter due to the long nights and insufficient cold, their existence would be highly precarious, and i expect it wouldn't be too surprising if the remaining australians would be evacuated to the safer new zealand if they ever made contact.
onto new zealand. on paper, their situation is much improved, though i think the fact remains that christchurch, with a population of almost 400,000, is simply too populated to avoid an outbreak of the rash, especially if the north island of new zealand is going to succumb. we've seen that denmark, even as the fourth nation to close its borders, lost all but the remote bornholm to the illness; new zealand, i think, would be no different, with only the icy spine of mountains across the centre of the island providing shelter for the more remote, rocky western half from the silent eastern zone. nelson as the capital is generous, though i think it's conceivable; if nelson, too, falls to the rash, greymouth, population 10.000, or queenstown (NZ), population 16.000, would likely be the heart of the remainder of the island nation. invercargill, with its winter mean low temperatures hovering around freezing, would be an easily achieved reclamation.
but on all counts, the coastline would always be a risky proposition, because of the large population of fur seals that, as beasts, would threaten the small remnants of civilization. the paucity of snow and ice might mean that only small settlements of immune humans would live on in mountain settlements, like queenstown (NZ).
japan is a really interesting case. i'm still thinking about it, but i have a couple ideas i'll throw out in a couple days or so.