I know some very clever strata managers but I doubt if any of them would say definitively that dame and water damage was an internal problem, just from looking at pictures.
I would take one last run at them and tell them you want a building consultant to come in and look at the problem and see where it begins. It could be anything from an internal pipe in the wall to a missing roof tile.
If they refuse, take them to Fair Trading and demand that they fulfill their obligations under the strata Act to maintain and repair common property (ie, your wall). You’ll find the appropriate form HERE.