For Victoria when it states “Interior face”, that includes all the interior surfaces of a lot’s airspace including its outside airspace areas. You have to think of a lot’s outside airspace areas, such as a terrace or a balcony, like a bathroom. All the walls and floors surrounding a lot’s outside airspace will be a lot responsibility to waterproof if surface waterproofing is used eg by a waterproof membrane.
But check the Plan of Subdivision to ensure the leaking wall actually is a lot airspace boundary wall.
If interested, it’s the Subdivision (Registrar’s Requirements) Regulations 2011 VIC that covers this area.
An excerpt:
<span class=”wcrep2″>(4) Unless otherwise specified on the plan, the location of any building boundary defined as—
(a) Interior Face lies along the interior face of any wall, floor (upper surface of elevated floor if any), ceiling (underside of suspended ceiling if any), window, door or balustrade of the relevant part of the building. Any internal coverings, waterproof membranes and fixtures attached to walls, floors, and ceilings are included within the relevant parcel;</span>
“Internal coverings” doesn’t mean “inside coverings”. It means internal to the lot airspace.