There is reason to believe that Australo-Melanesians used to occupy a much larger area than they do today. Let’s define them by their genetic affinity to that odd genetic trace in Amazonian Amerindians: those related include Papuans, Aboriginal Australians, Andaman Islanders ( closest), and Negrito groups in Southeast Asia and the Philippines.
Skeletal evidence says that they used to occupy all of Southeast Asia (probably at least as far north as southern China, fairly recently), and have been largely replaced by people from further north over the past few thousand years. Same is the case for the Philippines & most of Indonesia.
In fact, even further north than that: a skeleton from Tianyuan cave, not far from Beijing, shows the same genetic trace. From about 40,000 years ago. So it’s more plausible that there was a potential source population in a more geographically felicitous area ( for settling the New World) at the proper time ( ~20k BC).
Looking at recombination (roloff) should tell us approximately how long ago the admixture occurred. I would hope to see rare mtDNA haplotypes in those Amazonian Indians – Y chromosomes, considerably less likely. I would expect to see a few alleles from the Australo-Melanesians favored by selection in South America. By analogy with what we’ve seen with archaic admixture, stuff like HLA would seem likely.