The Last Wave (1977) IMDb
Modern Sydney, Australia. A lawyer with weird dreams defends some “Aborigines” in a murder trial. There is precedent for tribal Australians being judged according to tribal law rather than national law, and the lawyer would prefer this, so he tries to get the accused men to admit that they are tribal, despite having modern, urban lives. They refuse, but the lawyer thinks they are hiding some connection to the ancient culture, and that he himself has a major role to play in that denied mythology.
Fantasy, clash of the rational/modern/familiar and superstitious/old/alien. Quite Lovecraftian.
References here: Cargo (2017).