One way trip so you will need to sort out flights back. Might be cheaper to book a return flight (Hawaii to Sydney, Sydney to Hawaii) and skip Sydney to Hawaii flight.
A pretty good deal IMO. I have been keeping an eye on this cruise for a few months. Works out to be $84.33 per person per night. That's unlimited food, accommodation, travel, entertainment. Won't beat that value living in Sydney. Food and service a bit better than Royal Caribbean cruise line.
Has many sea days. But might suit someone that really wants to relax and unwind. Unpack once, don't cook or clean. All done for you. No phone reception (unless you buy internet pack) which can be a bonus if you want to disconnect from work and social media.
I always thought that if you skip a leg, the whole flight voids