In my opinion the best mass market cruise line in Australia is Princess.
If you are looking for something a bit exotic, I would suggest a Papua New Guinea cruise from Brisbane. For scenery, PNG is like a tropical NZ, except that the best and easiest way to see it is by ship.
You can easily follow that up with a North Queensland cruise to see the Great Barrier Reef.
A bit left field, but you might not have thought of a visit to Lord Howe Island and/or Norfolk Island. Wonderful places that you can fly to from Sydney.