I had this too. OMF confirms there are no new govt regulations. I think it's this from https://www.fma.govt.nz/contact/faqs/#Foreign:

My foreign exchange broker has just told me they must close my account because of New Zealand regulatory requirements. Why?
Most foreign exchange brokers issue derivatives. If they are offering derivatives to retail investors they must be licensed by us – even if they are making an online offer from outside of New Zealand. Some providers have applied for a licence but others have decided to leave the New Zealand market. This means they cannot accept new retail business from New Zealand and they must close all New Zealand retail customer accounts.
Why can’t I use an Australian regulated, European regulated, or US regulated foreign exchange provider?
You can, but it’s illegal for them to offer derivatives to retail investors in New Zealand without being licensed by us. If they are willing to break the law to get your business, it’s likely they will be cutting corners in other areas and you will have much less protection if things go wrong.
Generally you’ll be a retail investor unless you meet certain criteria, for example, you’re a family trust or you have a large sum of money – in which case you could be considered a wholesale investor.
If you are a wholesale investor, derivatives issuers can continue to deal with you without being licensed by us.
We recommend you only use providers licensed by us, even if they are or appear to be regulated overseas. It’s hard to distinguish between genuine and fraudulent overseas providers – putting you at a much higher risk of being scammed.
So unless we're a wholesale investor, or managed fund to answer your question, we apparently need protecting from ourselves. And the US brokers have obviously made the decision that this market isn't worth the time/money/hassle to get a licence from the FMA when the rest of the world is their oyster.