Sri Lanka is just off the coast of southern India. Then it is only about 600 miles east to the southern archipelogo of Malaysia and Indonesia. And that is only if you didn't want to start out by land from northeast India, trek across to Burma, down into Thailand and south into Malaysia and then across to Indonesia. Though, I expect a mostly land route might have (a) taken longer, (b) met more people already settled along the way and (c) experienced more migrants settling, or being killed somewhere before they ever got down to Indonesia and Australia.
It is even less difficult after crossing the ocean from Sri Lanka to just island hop down the archipelogo of southern Malaysia and Indonesia until you are hopping along very southern Indonesia and in the Timor Sea. That leaves a route of less than 250 miles south across to northern Australia. Don't ask if they just got across from Indonesia to Australia by accident - taken that way by oceans currents??? - or if someone had once made it back to them from such an accident and informed them of a big land in the "southern sea", and remembered how to get there. Who knows??/p>
Given what I see as the ease of it, there must have been a lot to the living context in that part of Asia that made such travels/migrations less frequent than they seem to have been - or maybe a great many of such travellers either did NOT survive the journey or did not survive in equal numbers in barren northern Australia. We can only wonder.
It just seems that the journey was not extremely difficult, but an 11% contribution to what is itself a small gene pool suggests there was either, for whatever reason, not a whole lot who undertook the journey, or not whole lot who had great success in it.
I am sure many prognosticating cultural anthropologists are about to tell us they know why the migrations from the Indian subcontinent to Australian were as small as they seem to have been. I never accept their guesses. I have no problem leaving unanswerable questions unanswered.
I know it is a much later date[3rd to 11th century], but to the uninformed, Madagascar was discovered/inhabited by Malay/Indonesians!