I have cruised to New Zealand quite a few times (as well as land tours) on Princess, Celebrity, Royal Caribbean and Holland America. I have found HAL to have the most varied itineraries and value for money. As mentioned in a previous post, if time limited, do New Zealand and save Oz for a later land tour (if possible), self drive would be the most economical as lands tours in Oz are ridiculously expensive.
A number of cruise i lines start Auckland, finish Sydney or vice versa. This eliminates two return sea days which, IMO, are a waste.
I thinks CCC's recommendation is worth considering.
"So much to do, so little time" - Anon🙂