A few years back I was discussing Denisovans in Wallacea: here’s the map again.
Cooper and Stringer were suggesting that Denisovans had colonized some or most islands in Wallacea, and that explains Denisovan ancestry in populations there today (Philippine Negritos and Melanesians). I thought it a bit unlikely because it meant that Denisovans had to have had seagoing technology, but in a very narrow range: good enough to reach Wallacea (and the Philippines), but not good enough to reach Sahul (Australia/New Guinea). I was wrong: it now looks to be true. Since that article people have found good evidence of early settlement in the Philippines , also in Sulawesi, as well as Flores. Some of those are older than Denisovans, likely erectus.
Look at the glacial-max map of the Philippines and environs, when sea level was 120 meters lower than today:
You can’t walk to the Philippines, but the water gaps are not very wide, on the order of 10 kilometers. I don’t much believe in breeding colonies being founded by tsunamis – note that only a few placental mammals ever succeeded in crossing, as pointed out by a commenter – but I can imagine someone paddling a log across those narrow gaps. The required crossing for Sulawesi is also narrow.
Elephants are pretty good swimmers and can cross that kind of gap, and various kinds of elephants colonized the Philippines, Sulawesi, and Flores. There’s a known example of an elephant that swam ashore after being washed overboard 48 km off the South Carolina coast in 1856.
But why didn’t early humans make it to Australia back then? Two thoughts: that would have required multiple sea crossings, some considerably wider that the ones leading to the Philippines or Sulawesi. Maybe that was enough to prevent it. Second, some of the islands in the proposed paths were small. Even if some population managed to settle one, it may have been too small to succeed for long. There was an island between Tasmania and the mainland that was inhabited for a few thousand years after rising waters cut the connection with the Australian mainland, with a population around 400. And then it wasn’t inhabited anymore: something happened.