I think when you say Australoid, you mean Veddoid, and Vedda people are distinct and different from Dravidians, including Tamils. Tamils have a range of phenotypes. Tamil-speaking groups can range from very dark with low nose bridge and broad noses to dark, normal nose bridge and average nasal index.
Whatever the Tamils may appear to be, when we put Tamil skulls on a graph, they plot with the Senoi, Negritos, Andaman Islanders, Papuans and similar groups.
The Senoi are an Australoid group that are best seen as Veddoids transplanted to Southern China and then to Thailand 5,000 YBP. From 5-15,000 YBP, Veddoid types may have been widespread throughout Asia. The pre-Jomon in Thailand 16,000 YBP have skulls that plot closely with the Jomonese later in Japan at 13,000 YBP. If you compare photos of modern day Ainu people with Veddoids from India, it is clear that the Ainu are a depigmented group of Northern Veddoids.
There are also traces of Veddoid types in the Philippines long ago where reports of oddly Caucasoid-looking people appear in the anthropological record. They may have been part of the group that moved from Thailand to southern Japan between 13-16,000 YBP because the Philippines would be along this route.
So also would be Australia.
And indeed, the first modern Aborigines appear in the form of a group called Murrayans that arrived between 15-20,000 YBP. The best guess is that the Murrayans were the same group of Veddoids that were present in Thailand at this time and the migration may have part of the same Thailand-Japan migration that the pre-Jomonese undertook.
This is probably not the only Veddoid migration to Australia. Between 10-15,000 YBP, a group called the Carpinterians arrived, apparently from Southern India. Consider that all people in India are termed “Australoid” before 8,000 YBP when the Australoid-Caucasoid transition begins possibly with the arrival of the first actual Dravidians, probably Elamites from western Iran. These pre-8,000 YBP Australoids in India then could probably best be called Veddoids.
Australoid types were present long before in India in India and Southeast Asia as skulls from India and Thailand 25,000 YBP are said to resemble Aborigines. Their relationship to the Australoid Veddoid group that shows up at least 16,000 YBP is not known.
The modern Aborigines are a mix between Carpinterians and Murrayans that arrived 13-17,000 YBP in the subcontinent and subsequently interbred.
Before that, some very different and even more archaic people lived in Australia.