Princess, Celebrity and Royal Caribbean, as well as some other lines, all have ships positioned in Australia during the southern hemisphere summer, generally November through to March.
We did a Princess Sydney to Auckland cruise 16 yrs ago but organised our own long haul and internal flights along with Barrier Reef, Ulhuru, Sydney and Auckland highlights through Trailfinders, and very enjoyable it was, despite BA c*****g up our outbound flight.