For the international audience, that UK assumption of "Terra Nullus" was knocked back and overturned by the Mabo decision of 1992:

    In Mabo v. Queensland (No. 2), judgments of the High Court inserted the legal doctrine of native title into Australian law.

    The High Court recognised the fact that Indigenous peoples had lived in Australia for thousands of years and enjoyed rights to their land according to their own laws and customs.

https://aiatsis.gov.au/explore/mabo-case