Every cruise requires some proof of Australian Citizenship before they allow boarding. While a passport is not required for the some cruises a notarized copy of a birth certificate with a raised seal and picture identification is. No, you may not use military ids, voter registration cards, or a library card. You must have a notarized photo copy of your birth certificate with a raised seal if you do not already have a passport.
Some ports even require a visa but the cruise line will inform you on proper documentation in plenty of trim prior to travel or even final payment.
It's a sad, sad, sad thing to see someone denied boarding for lack of proper documentation. Make sure you have your documentation well in advance. Don't assume it's in your safe the night before your cruise. Look for it by the time you make your final payment.