Just found this in the famous Who’s Responsible Memorandum put together by the NSW Lands office and Fair Trading (and many others) to clarify disputes:
2.13 General – OC responsibility
h. TV aerial and associated wiring (servicing more than one lot and regardless of whether it is contained within any lot or common property).
What does this mean in this case? Firstly, the Owner’s Corp were duty bound to fix the problem. However, knowing you were going to demolish that wall, they should have alerted you to the fact that there were common property cables in it (if, in fact, they knew themselves). I’d be going to mediation with a view to sharing responsibility with them and splitting the cost accordingly.