That happened to me and my husband a few weeks ago. We’ve been doing our 90 day online for ages. Nothing different, but got rejected. We tried three times. We ended up going into Immigration. No explanation or reason 🤷♀️
To be fair, there have been a few flights, but very expensive, or transiting through countries that end up closing their borders. And now, NZ have put a quota on the numbers of returnees. This is our reason for wanting to plan ahead with a flight booking. 🙂