Some cruise ships I have been on specified the dress for certain evenings. "Formal" would indicate a Price Charlie. If you are uncertain, an Argyll would be a good choice. As for the Sydney Opera House and a musical, I don't know what is usual there. This evening I went to a performance of the Austin Symphony and the attire was all over the place. In a couple of weeks I am attending the opening night of the Austin Lyric Opera and I am wearing a Prince Charlie jacket with my kilt. Undoubtedly there will be some people attending in t-shirts and jeans.