What would a Southeast Asian-Aborigine culture look like?

More of a culture worldbuilding thing, and less of an AH, but it could be tied in if nessecary. You could have a Indonesian-based empire make colonies in the south, but what I really seek is what will be some points of cultral assimilation, cultral exchange, etc.
Also when I say Southeast Asia, I am being very broad here, but for Australia I mean the north.
 
They could spread Buddhism to Australia, certainly, but they'd need to gradually incorporate the Aboriginals into their trading networks and such to get any real result and real cultural fusion. The main centers would be along the East Coast since the soil isn't terrible by Australian standards and the climate is wet enough. Australia is so big and dry that it's unlikely they'll get to Western Australia anytime soon, but they might push south at some point when they find sandalwood there. The secondary center would be along the rivers in the Top End and the Kimberley, which due to climate and soil won't be as productive. Australia would be considered part of that general cultural area and possibly speak Malay as a lingua franca.
 
When? Why? Which "Indonesians"? There are numerous highly divergent scenarios that could be encompassed under the label "Indonesian-based empire w/ Australian territories". The only OTL example of significant Indonesian contact with Australia was the trepang industry, which depended on demand from Qing China. This was ultimately short-circuited by the rise of colonial Australia and nevertheless occurred during a time in which Indonesia was already coming under Dutch domination, although I suppose that could have been stalled up to and around 1830. An early trepang industry is certainly possible, depending on Chinese demand, but I really don't know enough about 17th century and earlier internal Chinese markets and their external demand for sea cucumbers to guess how likely this is.

Beyond trepang however there is no evidence of any significant Indonesian contact with Australia. Why then might this change? Northern Australia is the only readily accessible part of the continent and it has an unforgiving environment, fiercely defended by Aboriginal people. In turn however, the economic base of Aboriginal Australia was at best highly dispersed in the north and did not produce in significant quantities goods that are likely to be in demand in Makassar or Java, even if they somehow know about them. I think there'd have to be major changes for there to be an economic incentive.

The other obvious reason for Indonesians to visit Australia in force is imperialism for its own sake, or perhaps a religiously-inspired invasion. This would almost certainly end catastrophically. While militarily the Indonesian states would be able to defeat any of the Aboriginal nations as far back as two thousand years ago, even getting to Australia is a major effort with pre-modern ships and supplying any expedition would be expensive and difficult in an alien environment. If you're based in Java and have the resources to invade distant lands, better to go north or west. South, and indeed east, has limited prospects.

Finally the "who" is a pretty big deal. Is it Srijaya or Majapahit? Or is it some smaller polity? Indonesia has hosted many maritime powers, but it is an extremely difficult place to build a lasting empire. One with the resources to reach Australia, in particular in earlier times, is also one that is likely to need those resources to just hold itself together.

IMO the best bet remains an early trepang industry that slowly morphs into a larger economic and eventually political presence in northern Australia. Perhaps getting some other group in on the action to the Makassans in order to foster competition and investment might speed things up. Banten maybe? How the northern Aboriginal nations react to these changes is a major factor, but one hard to gauge. The Makassan-Yolgnu relationship is the only example we have of a semi-equal relationship between an Aboriginal and non-Aboriginal group and while it appeared to be largely peaceful and to foster many major changes in Yolgnu society, an actual Indonesian presence on the mainland would be a different story entirely.
 
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