Couple of simple solutions as I see it.
1) Any new development must install cable etc. If TPG can do cable to a building, there must be a market in cable to a development. If it becomes more attractive for them to do any other established streets along the way so be it. To allow for payback whoever does the cable gets a monopoly on the install for a period of time (subject to a maximum price) before the asset is turned over to the NBN.
2) Allow existing developments to be changed over to cable in the same way outlined above. Developed areas with the poorest access would soon become the best. Again after a period of payback the assets would turn over to the NBN should it ever develop.