International Travel Charges
A number of airlines are now only (or soon to be) allowing for one 'free' piece of checked in luggage per flight. Extra baggage will cost more and there has been concern expressed at the cost for this extra piece. A $200 fee applies if you turn up to the airport on the day with an additional bag. In your case the firearms would be counted as an extra piece.
Bookings made after the 22nd Of November for flights on or after the 27th of November will now incur the following ADDITIONAL charges if pistols are carried separately as luggage:
Australia $120 per piece, rest of the world $200 per piece.
Check out the following link: http://www.airnewzealand.co.nz/excess-baggage-nz ; for more details and remember any flight information is subject to change so you should check before you fly.
Here is the Air New Zealand webpage for cheaper rates when pre-paying