By this time rising ocean levels had made the narrow channel between Sundaland and the New Guinea/Australia landmass too wide for easy access.
Well there are tribes in the Andaman & Nicobar Islands of India, where the people there supposedly have African origins, and are completely unrelated to the people living in the Indian mainland.
http://en.wikipedia.org/wiki/Andaman_and_Nicobar_Islands#Name_origins