Ahoy there, Sab! Welcome!
First, I hope you have an absolute blast on your upcoming
Disney Cruise Line adventure on one of the brand new Australian itineraries. Those are shaping up to be some truly memorable voyages, and I'm so glad you are going to be able to enjoy one with your family. Your particularly
epic itinerary not only gives you some awesome sea day opportunities to explore the
Disney Wonder, but you also have that port stop in
Nouméa, New Caledonia. Amazing.
Okay, so the short answer is yes, residents of Australia do need a passport to travel to New Caledonia. The closed-loop cruises you've read about are only for cruises sailing from a US port and returning to that same US port. It's part of the Western Hemisphere Travel Initiative, which was put into place by the US Departments of State and Homeland Security, and it applies only to travel in the United States. It doesn't allow US citizens to travel anywhere without a passport, and even on a closed-loop cruise, if you are traveling to a country that requires a passport, you will need one to get off the ship.
Having said that, even with closed-loop cruises, although a passport is not part of the
required documentation, it is highly recommended. There's a whole bunch of red tape and possibly some additional expense if you have an emergency and need to get back to the US from a foreign country without a passport!
Sab, I hope this clears things up for you! You should be shipshape and ready to sail!
Anna