Re-read both Skillet and BearPaw above. I am also a known shipper, and I lived in NZ for several years. Their information is spot on!
My strong advice is to try to prep your skulls and take them back with you as personal luggage. Yes, you will pay extra, but the amount that you pay to do this will be significantly less expensive than having a freight forwarder ship them for you. The added benefit (and this is huge) is that you get to speak directly with US CBP at the airport upon arrival.
If you do your skull prep correctly (...no flesh..., completely dry, &c.), then your chances of getting through Customs at the U.S. port of arrival are very good.
I will also hunt the South Island, maybe next year. Somewhere around Te Anau. I would really like to do an extended trip to Secretary Island, but that is a bit of a stretch. The mountains around Milford Sound are also enticing. But you don't have to go that far. The South Island (the north-south spine of the Southern Alps) are great. Find a local guide--they are cheap!!
DM me with questions anytime.