There are several options to get down here from your part of the world, pending the retiree agreements you have with OAL's. More obvious options would be SEA-YVR-SYD on AC, SEA-SFO/LAX-SYD on UA, SFO/LAX-SYD/BNE-CNS on QF, YVR/SFO/LAX-AKL-CNS/SYD on NZ and then the Far East carriers.
How easy it is depends on the day of week and time of year. There are very defined peak seasons for flights into/from Oz and in those seasons flights are packed.

P&O cruises Australia (sister company to Princess) operate 2 or 3 vessels from various ports in Australia year round. Variety of options to choose from - round Oz, South Pacific etc.
If the time of year is right (very broadly NOV-APR) then some of the more familiar names cruise around down here, either as segments of world journeys or dedicated short cruising.
Royal Carribbean, Holland America, Princess, Cunard, Crystal, Silver Seas etc.