Agreed. People didn't just send boatloads of settlers to random lands they happened across and established New X state here like a video game.
Did they need new lands so badly and so far a way that there would some financial pay off? Were there easily available resources that were also easy to discover, and scarce at home ports, to also cause investors to spend their precious coins on a gamble rather than on reliable trade routes that had already made them fantastically wealthy?
Remember, Europe was largely isolated from the Asian trade networks, causing there maritime ventures. Kilwa was already a valuable middle player. Plus, East Africa had been a part of this trade network since antiquity, unlike Australia, so the comparison is pretty terrible in justifying a Kilwanian colonial empire.
Also the British only began settlement in order to have a secure port in which to project their power further into East Asia and the Pacific. They also had a convenient penal population that needed a new place to be shipped to with the loss of the States. So in conclusion the UK and Kilwa were in completely different places at their respective points of development when they could/did have the opportunity to exploit Australia.
Kilwa would have needed to have survived, grown in power as a main contender of Indian and Pacific Ocean trade to ever have a need to colonize another continent. Don't mean to rain on your parade, but these things need to be considered when launching polities into European-style maritime colonialism.
And this as well. I will note that it is easier to sail down the coast to Sofala from Kilwa than it is to sail to Australia.
There are a number of prohibitive factors here. Firstly, Kilwa's Perso-Arabo-Swahili upper crust actually benefited from the existence of local kingdoms, because the Zanj, in addition to spices, was the prime locus for the Indian Ocean slave trade, which Kilwa and later Zanzibar grew off of. Local kingdoms spared Kilwa the administrative costs and allowed for a layered trade network, not unlike the small trade settlements that anchored the Mediterranean trade networks of the Italian city states.
Secondly, those coins were almost certainly deposited there by Malays. There may well have been other coins, but they were probably lost or used elsewhere; the location of the coins doesn't exactly sound like prime real estate. Malays and Indians came to Kilwa, and some Kilwans went to Malacca and the East Indies--they didn't need trade settlements because they were already part of a seamless network, unlike the Christian-Muslim competition in the Eastern Mediterranean.
Thirdly, the distance is totally prohibitive. They'd have to travel up the Horn, to Gujarat, down the Indian coast, up the Indian coast again or across to Aceh if they were capable, before going the length of the Indies to get to Northern Australia. Nothing in northern Australia is worth that kind of expenditure.
Fourthly, trade-motivated people don't tend to settle empty lands that are far away and require intense expenditure just to develop. Why did the VOC only settle 2,000 people from the Netherlands in South Africa over the course of their rule in the Cape? Why didn't the Spanish settle more deeply into North America? Why didn't the Dutch or Portuguese settle Australia before the late 18th century? Because they were there for spices and Christians, not land to settle. The largest settlements were those in New England and English North America. Why did so many Englishmen--including the English plurality in New Amsterdam--settle in the Americas? Because they were there, not to trade but to build a New Jerusalem and escape persecution. Without something overriding Kilwa's supposed profit motive, they have no reason to settle a place as forbidding as Australia. Trade requires locals to trade with--there's a reason Portugal went for Ceylon and Malacca.
Fifthly, if they actually wanted to settle (they didn't), there's either Madagascar or the Cape available, much closer when one factors in the fact that they can't sail in a straight line from Kilwa to Darwin.
Sixthly, there is also an assumption that the Kilwan state is centralized enough--or that the elites have an interest in--colonial exploration. The particularities of Europe's earliest missions were borne of European isolation and the specific European class and legal structures that required the resources of the Crown to go out into the great unknown. 13th century Kilwa is not going to have the resources of late-15th century Castille.
Seventh, competition. If, for some reason, Australia actually became a compelling place to settle (it wouldn't), then Kilwa is competing with not only maritime powers like the Chola but local powers like the Majapahit as well.
In short, without Europe's burgher class and the pressing issue of trade isolation and the aftermath of the crisis of the 14th century to compel them, there is no reason that the Kilwa Sultanate would have the means, resources, or, most importantly, need to go settle in northern Australia. Europeans were the ones that turned the Indian Ocean into a battleground for major competition; Kilwa, as a major node in a mostly-stable network, has no reason to go settle someplace that wouldn't even afford them greater access, to, say, the Moluccas.
