Australian members may want to comment, but from my experience of visits down under, a sports jacket and tie counts as rather formal dress in Australian culture. A few people were wearing suits and ties at the opera but I suspect unless it is a gala night, formal is not what it means in Europe or the US.

An Argyle with a kilt is going to put you at the very smart end anyway.

I would not want to comment on the ship except to say that in a mostly unkilted company, simply wearing a kilt with a proper jacket (not necessarily black with silver) puts you in the smarter crowd anyway and no one would suggest it is not "formal". But you have to be comfortable with your look!