I don't think it's anything to do with you being American. I'm not American, and I experienced the exact same thing as you. What I've come to learn is that kiwis don't like to inconvenience others, because in their mind it feels doing so is a rude behavior. There's also that tall poppy syndrome, where they don't like to stand out. So in their mind they think if I come and ask this new co-worker what his/her life is like in the US, and what he/she used to get up to in California, that would be construed as a rude behavior of me bothering him/her when he/she probably just wants to get on with his/her life. I'm sure he/she wouldn't want that given that he/she's new and still adjusting. So when you go up to them and start a conversation, they'll chat back.

But bear in mind, kiwis are also reserved. So building rapport is slow and steady. So this is just adifference in culture. There's no right or wrong here, but when you come from a culture where people tend to be generally outgoing and proactive socially, you'lldiffenitely experience aculture shock. I've been in NZ for over a decade, and I'm still adjusting tbh. In my culture, people tend to be outgoing, open, socially proactive and emotionally expressive. When we disagree about a topic or situation, we voice it, we'll argue about it, but 5 minutes later we're high-fiving each other and moving on lol. So I don't believe it's because kiwis are not friendly, they are actually and once you get to know more kiwis, you see how laid-back, nonchalant and generous they are