It’s the “Exterior Face” that is your issue.
If that is declared on the Plan of Subdivision, then the wall is totally your responsibility to maintain. Right through from its interior surface to its exterior surface.
Parts of it that are load bearing to the building would be common property. But in many types of construction, the exterior walls themselves are not structurally load bearing to the building.
While “Exterior Face” is not exactly common in VIC apartments, it’s not rare either. So lot owners can indeed own external walls or parts of them.
Even if the wall was the more common “Interior Face”, you’d still be responsible for any waterproofing membranes under its exterior surface. That’s legislated in VIC.