Would this be seen as a racist thing to do?

It's not racist, just in bad taste if someone from PNG was sitting there. In work the saying is "Intent is irrelevant" meaning it don't matter the feeling about it, certain things can be offensive - but only if the victim is there. Now, it's probably something religiously based, the outfit. And a more relevant offensive exaggeration would be like if I wore a yamulke over my whole head, and two braids of hair down to my waist, happily saying I was a Jew. But figuring no one aborigine is gonna be there, and it's a less known culture, who really cares?