It can't be equipment so I am guessing that AAC-HE is a mandatory audio codec for NZ DVB-T transmissions but DD is optional. And perhaps they won't to carry another audio track in order to save bandwidth for all their other DVB-T channels like TV6 and TV7.
OTOH TV3 only has one channel (or is two?) and so have some bandwidth to spare and therefore don't mind carrying both audio tracks all the time (thank goodness). Of course when their next channel goes up, who know what they will be doing.
Until then I guess I will support TV3 (they have the best OTA programming anyway) and only rarely watch TV1 and TV2.
What are others doing? I am guessing those with working HTPC's that can handle AAC-HE audio or have purchased DVB-T tuners or have TV's tuners built in don't care so much but even then, the difference between a good 5.1 track and stereo is notable.