Good news everyone!
The NZ Buildings data is now available to be added in RapID: https://mapwith.ai/rapid
Click Rapid → View/Manage Datasets → Navigate to add the New Zealand Buildings layer. (Currently it is on 11th page there)
I suggest comparing the building outlines to the most recent aerial imagery available for the area, and making any changes/refinements required, before uploading. In my tests this was typically for large renovations which have extended house footprints, full building rebuilds, and occasionally ignoring buildings which have been entirely demolished.
The buildings can also be added using the MapWithAI plugin for JOSM: https://josm.openstreetmap.de/wiki/Help/Plugin/MapWithAI
All credit goes to Timothée Duhamel from Talk-NZ mailing list.