Just to add husband and I are planning to use Alaska Air miles with the same plan to fly to Sydney or Auckland for a cruise. Easy pickings for getting the credit card more than once in a 12 mos period even 3x with bonuses from 25k to 30k depending on application.
Too early for my flights for 2018 but you can play around with the site and see that you can fly in mixed cabin ( bus & coach or premium economy) with only 55k miles ( one way)
I too am planning on using my Citi Thank You Premier pts for return flight in economy.If you go to cruise critic forum there is a thread called Australia and New Zealand cruisers who may help you with places to stay.

http://boards.cruisecritic.com/forum...&daysprune=365