I can give this one a crack,
There was evidence of widespread trade between the Austronesians of the Indonesian archipelago (especially the Makassan states) and Aboriginal tribes in Northern Australia, with almost industrial scale farming of the sea cucumber for sale in China, where it was considered a delicacy and so was quite expensive. This trade and contact can be felt in the Aboriginal languages of that area, with words such as rupiah for money, indicating a knowledge of currency.
However, large-scale settlements would be extremely difficult for two main reasons-
Australia was inhabited already, while Madagascar was not. Especially considering the higher density of Aboriginal populations along the north coast of Australia, colonising and subjugating an entire massive area would’ve been too steep a feat for pre-industrial Austronesian states. There is discussion whether there were small Makassan villages established along the coastline, but even if they existed they didn’t last long, due to food shortages, local resistance, harsh terrain, etc.
Indonesian ways of agriculture, especially in the larger more organised states, were far too different to be transplanted onto Australian soil. Consider this- Madagascar is mainly plains interspersed between forests- not perfect for rice-based agriculture, but just fine for wheat, pigs, chickens, etc.. Northern Australia, however, is one of the most inhospitable areas to start a colony in the world. The industrialised colonialism-focused British could barely hack it near modern day Brisbane, which is much more hospitable. Northern Australia features-
-Arnhem Land, which is literally a jungle plateau.
-Cape York, full of rainforests and cyclones
The Kimberleys, an area of jungle so thick that there are no decent roads in or out today.
-Did I mention cyclones? Because there’s a lot of those everywhere.
Pretty much, those people at that time in that place just was never going to work.