So, here's the premise- in the mid 11th century, a Vellalar prince, looking to gain influence and power in the Imperial Chola court, funds and organizes a small colonial expedition to search for new unclaimed lands south-east of Java, attempting to follow the spice route back to its ultimate source; first establishing a trade post on the island of Sumba in passing, but then pushing onward in a south-easterly direction, bypassing the Savu Sea, and discovering North-Western Australia in the process (making landfall somewhere along the coast of the Joseph Bonaparte Gulf).

And over the course of the next few decades, he subsequently follows the standard MO employed by the Vellalar across South India, Sri Lanka and Sumatra- wherein settlers would move in to colonize virgin land, which had previously been used by hunter-gatherer tribal peoples only for slash-and-burn agriculture and/or hunting, and convert it into prime agricultural land (with 'Vellalar' being an honorific title, granted to a select few people who would organize such raids and establish such settlements, with the chiefdoms themselves also known as 'Vel', and the title of 'Vellalar' historically being restricted to the heads of these villages, or the aristocratic lineages of their founding chiefs).

BTW, the primary narrative viewpoint (of the 1st installment in a planned fictional family saga alternate history series, drawing inspiration from the works of Edward Rutherfurd and James Michener) isn't going to be that of the aristocratic leader of the expedition himself, but instead a member of the Setti-Guttas, one of the Chola trade guilds, which was famous for specializing in the field of mercenary warfare more than actual merchant activity, with its members most often hired by traders to ensure protection of itinerant groups and caravans, as well as to ensure the safety of trading settlements.

And these pioneers focus on native sandalwood (Desert Quandong), camphor, cineole (eucalyptus oil), coral, pearls, and rudraksha prayer beads (seeds produced by tree species in the genus Elaeocarpus- with the largest and most colorful of these just so happening to be produced by trees native to NW Australia- which were deemed sacred to the god Shiva, whose temples were also established and utilized across the Chola Empire by the Trade Guilds, and effectively doubled as the Guilds' corporate headquarters) as the most lucrative commodities to exploit in establishing the first permanent colonial trade settlements.

How successful do you imagine that the Tamil Chola settlement of Northern Australia (perhaps expanding outwards to influence the entire continent as well, in later generations) could possibly be, then, without stretching the bounds of plausibility too far- for instance, in a best case scenario, could Australian kingdoms and/or empires conceivably arise which are more populous and/or powerful than those of the Indonesian archipelago, or even than Japan? And by the time the first Europeans arrive in the region, how radically different would you expect this world's Australia (or rather, 'Kumarikkandam') to be, compared to the Australia that the Europeans 'discovered' in our own historical timeline?

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    $\begingroup$ Sweet premise but you need to close with a discrete question. As it stands your closing query sweeps across the entirety of your endeavor. Narrow it down please or it will get closed as too broad. Vote to close, and also upvote out of hope! $\endgroup$ – Willk Nov 20 '19 at 18:52
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    $\begingroup$ They don't need mass irrigation. The heartlands of the Chola Empire have the same climate as N. Australia, with the same tropical wet and dry/savanna climate, and similarly long dry seasons, along with slightly less annual monsoon rainfall, and slightly higher extreme temperatures and humidity. As such, their farming techniques and crops were already perfectly suited to this region of Australia- e.g, black gram, which gets rotated with rice in the paddies of Southern India, used both as an equally important food crop and to enrich the soils in the process through its nitrogen-fixing. $\endgroup$ – Aquar1animal Nov 20 '19 at 19:04
  • $\begingroup$ Where can they get the water for the mass irrigation needed to supply the farmlands necessary for any kind of big population in Australia? You need to narrow your question to deal with a single issue. As the saying goes "from water does all life begin". Maybe a good place to start would be asking about that. $\endgroup$ – Bitter dreggs. Nov 20 '19 at 19:05
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    $\begingroup$ Why, Monica? Just compare the climate charts of Chennai and Darwin. Chennai has a record high temperature of 45°C, average high of 32.8°C, average low of 24.6°C & record low of 13.9°C, with 1392mm rainfall per annum, 59 rainy days a year, relative humidity of 70%, and 2762 sunshine hours. Whilst Darwin has a record high temperature of 40.5°C, average high of 32.6°C, average low of 23.2°C, and record low of 10.4°C, with 1720mm rainfall p/a, 113 rainy days a year, relative humidity of 53.4%, and 3092 sunshine hours. Chennai manages to support a huge population- why shouldn't Darwin be able to? $\endgroup$ – Aquar1animal Nov 20 '19 at 19:13
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    $\begingroup$ Also worth mentioning, for reference, regarding the question of water supply- the largest river which flows into the Joseph Bonaparte Gulf is the Victoria River, with an average annual outflow of 5,000 gigalitres, or 1,584m3/s. This may not be much by global standards, but it's still almost the the same as that of the Krishna River, 4th largest in India; and 2.3x the outflow of the Cauvery River, covering roughly the same area as the Cauvery basin- better known as 'Chola Nadu', since this basin & its irrigated agriculture formed the cultural homeland and political base of the Chola Dynasty. $\endgroup$ – Aquar1animal Nov 20 '19 at 20:03