I reside in the US and am planning a trip to NZ. Since I am a 1K (*A Gold) in United Mileage Plus, I am planning on using some upgrades to get to Australia, stopping first in Sydney, and return from Melbourne to the US. I would be doing a side trip with stops in Auckland, and Christchurch, from Sydney, and then returning to the US.
I was checking google flights, and found some very reasonable fares on Air New Zealand, and when I clicked the link from google flights, I was directed to the New Zealand site for Air New Zealand. I did a comparison and logged into Air New Zealand directly, which took me to the US version of the website. On the US version, the fares for the same flights are about double the price, not even accounting for the currency exchange.
Is it allowable for a US citizen to book flights on the New Zealand website? And will these tickets get issued?

FT has all the answers, so I await your wisdom. Thanks for all your input.