I agree with everyone who suggests that you should publicly post the address containing 0.6 BTC and thus not only try to attract the attention of the real owner, but also at least partially confirm your story.

@easternklaas, When I read your location, it occurred to me that a few months ago I read about floods in that part of Australia, so I wonder if it is possible that some of the floods actually caused that paper wallet to appear on the beach? I think that the flood hit some houses and thus took some things with it into the river that flows into the sea, which then threw that wallet onto the beach.

It's just a theory, of course, but somehow it seems more realistic to me than someone walking on the beach with a paper wallet containing 0.6 BTC, losing it and not trying to find it.