The response is here. While they haven't completed the privacy work yet, there is some very good news about data retention:
In regard to the third part of your request, the system is being designed to hold an image of vehicles for the period of time it takes to travel between cameras. A second image will then be taken and only images of vehicles that have an average speed above the speed limit as well as being above the operating threshold (same as static cameras) will be retained by the system, all other images will be discarded. Infringement notices for speeding offences which are recorded by the cameras will be issued by the New Zealand Police.
Which seems pretty good: no storage, no database, so no huge privacy issue. We still need to keep an eye on it to ensure that it isn't subsequently modified to do those things, but if this is NZTA's default implementation of ANPR speed cameras and traffic counting, then we should be safe.