How much does it cost to replace an iphone6 battery???

Sorry, I don't have any knowledge about Apple New Zealand but the following is a link to the iPhone battery replacement support site (same as the one I gave above, except for New Zealand rather than U.S.) and it indicates a price of NZ $85. See: https://support.apple.com/en-nz/iphone/repair/service/battery-power

$49 if by Apple in the U.S. -- see: iPhone Battery Replacement - Official Apple Support