I thought they were endemic to the mountain. If you are fish you surely have to go with the flow (down the mountain) or find steady water. I would imagine that these fish have been inhabiting pools of steady water for some time and have probably become sub-species over time.

Survival over the winter may not be a problem, as long as the water doesn’t freeze to the bottom of the pool the fish will do okay. The reduced metabolic rate at close to freezing means their food an oxygen requirements are minimal. We have a pond in our garden that freezes every winter and the goldfish are thriving.

How they got there to begin with? I suppose someone could have put them there, but there are two other possible explanations. If the pool feeds a stream, even intermittently, they could have migrated up. Another possibility is that either gravid females or eggs were carried there by wind since there are plenty of reliable reports of showers of fish.

Not knowing the area, I expected at first that the fish in question were killifish, some of which are famous for appearing in a small ponds, often temporary ponds that dry up annually. However, I checked and your Alpine ponds and streams in Kosciusko, not all of which contain fish, do collectively contain a surprising variety of species, some of them in surprising places. They include several species of Galaxias and a job lot of others, including at least two species of eels. The situation, already complex, is further complicated by the fact that some of the ponds are joined by streams when there is sufficient rainfall, which in Australia, I understand, is not everywhere every year.

Now, such small fish have a habit of appearing in surprising places. Sometimes it turns out that there has been human assistance. Sometimes it seems that birds and similar creatures have played various roles, according to the local circumstances. How often this is because some fortunate ancestor, or its eggs, got caught up in weeds entangled with the toes of an obliging bird, or because a passing tornado or fortuitous waterspout did a quick removal, no one can say. However, at least some of the Kosciusko fishes have been resident there for long enough to have formed distinct species. It seems that some of them at least can ascend, or even climb remarkably fast, steep streams. In fact that is how some of them seem to have survived the various trout thoughtlessly imported.

Good luck to those fish, say I. Some marvellous creatures have been exterminated by so-called angling fish released by persons who not only know no better, but do not want to know any better.

In Ireland we have trout (and other fish) in loughs that are isolated in the mountains, linked to the sea by rivers via impassable waterfalls. The explanation I've always believed is that they populated the water as the ice melted 13,000 years ago.

Perhaps birds are part of the answer.  Live fish may occasionally be transported in the gullets or feet of birds, and perhaps fish eggs could also be carried on feathers.