When Captain Cook "discovered" Australia in 1770 it was known that Southeast Asians and even Chinese seafarers had landed in Australia. Portuguese and Dutch ships had also stopped to replenish water supplies. It was not considered possible that people had landed in Australia from another continent.
In the 1940s mysterious coins were discovered. While some were coins used by the Dutch East India Company others were identified as coming from the Kilwas Sultanate of Tanzania in Africa. They were used in the prominent trading center 1,000 years ago.
If Africa was trading using Australia as a location en route Aboriginals could have come into contact with African merchants. Though the coins were found on the fringe of the Australian north coast, getting fresh water would have been important for ships traveling through the waters of the Wessel Islands.
Aboriginals would have been curious about any "strange" looking new people. Africans would certainly have been different. Drawings of European ships have been found on cave walls. They were drawn by Aboriginals after contact with Europeans. Perhaps evidence of African visitations can be located.