I did the sold a car form about 4 times for one, and it never went through.
I ended up calling nzta and they said yup we see you submitted it but it never went through (also we never contacted you to say that) and apparently that part of the system just doesnt work and no one has ever thought to do anything about it
Anyway. What you need to to is actually ring them and they will take it out of your name (always get the buyer to do it on the spot, but sometimes the phone system/internet is down at thatbtime)