At least with me, the problem ended up being weight, not space. So you might find that the space saver bags don't do much for you -- unless you've got a huge family and are entitled to a gazillion pieces of luggage -- and then I'm green with jealousy :-)
I didn't use any space saver bags or anything, just found the biggest allowable box (18 x 18 x 24) and stuffed my duvet in around the breakable stuff. I put my suitcases/backpacks (filled) in the boxes. If you do it this way, don't rely on tape alone - use twine and wrap it like ribbons around a birthday gift. (Learned this the hard way.)
Excess baggage charges didn't end up being that much -- cheaper than shipping via the US Postal Service. If you call AirNZ, they might give you an obscene number - $175 per bag was what they quotes me. When I got to the airport, I was only charged $80. But YMMV.
Anyway, good luck! Make sure you weigh each bag as you go - 70 lbs comes up fast!
Laura