Looking at specs of the Redback (RADIUS) it looks like unicast and multicast have different accounting attributes, therefore Telecom or any ISP for that matter, would in theory be able have differential rates.
BTW. if you really want multicast widely available within NZ - you need to help Telecom with a business offering first - a residential service would surely follow :-) Perhaps an ISP or national/global corporate could put their thinking cap on and convince Telecom of how much money they can make (or save) together...
As already said - the technology is there - there's just no guaranteed revenue stream. One good start would be having a multicast-only stream (albeit limited in content) available on JSG to enable users to set up their clients properly - most users don't know or even care if it's unicast or multicast, let alone how to get multicast going.