I know with previous versions you could buy an unlocked iPhone instore with no contract.
The website seems to insist on using a payment plan.
Can you buy an unlocked iPhone 6s outright in a vodafone store?
Navigate there via the "browse all mobiles" pull down, rather than the "Iphone" one
then you can get to this, with the "mobile only" option
https://www.vodafone.co.nz/shop/mobileDetails.jsp?skuId=sku880567&voucherCode=&selectionKey=mobile&menuKey=
