I've heard this secondhand, so apologies if it's not true and I'm spreading misinformation. But what I was told is...
NZ apparently went harder earlier on their repatriation messaging and Govt support for repatriating the Kiwi diaspora. More flights earlier to get people home prior to their big lockdown. Very few options to fly back direct to NZ now in comparison to Australia (which is obviously still very limited). Of the options that do exist involving transit, most will require some quarantine along the way anyway. Eg, may not be able to get directly to AKL but can get on a flight to SYD. Will do so, sit out quarantine in Australia and then find an option home. By the time they get to NZ, even if they were infectious they aren't any more.