If you want the short answer, it is the Owner Corporation.

To be sure of this for your building you should double check by reviewing two documents:-

The registered strata plan

What to look for: you are looking for a note on the strata plan that says something like the following

  • “the lot is responsible from the upper side of the slab.”
  • “The lot is responsible for the membrane…”

The Registered By-Laws

What to look for: a by-law that makes the membrane/waterproofing the responsibility of the benefitted lot owner.

Now if neither of these notations or by-laws exists, it is most certainly the owners corporation cost to access, repair and fix the damage caused.


If you want to understand why follow this investigation flow

What does the strata act say:

1. Strata scheme management act 2015

Definition

common property, in relation to a strata scheme or a proposed strata scheme, has the same meaning as it has in the Strata Schemes Development Act 2015.
Let’s look at the strata development act says then

2. Strata scheme development act 2015

Definition

common property, in relation to a strata scheme or a proposed strata scheme, means any part of a parcel that is not comprised in a lot (including any common infrastructure that is not part of a lot).

3. Check what “lot” is defined as

lot, in relation to a strata scheme, means one or more cubic spaces shown as a lot on a floor plan relating to the scheme, but does not include any common infrastructure, unless the common infrastructure is described on the plan, in the way prescribed by the regulations, as a part of the lot.

4. Understand what common infrastructure is

common infrastructure means:
(a)  the cubic space occupied by a vertical structural member of a building, other than a wall, or
(b)  the pipes, wires, cables or ducts that are not for the exclusive benefit of one lot and are:
(i)  in a building in relation to which a plan for registration as a strata plan was lodged with the Registrar-General before 1 March 1986, or
(ii)  otherwise—in a building or in a part of a parcel that is not a building, or
(c)  the cubic space enclosed by a structure enclosing pipes, wires, cables or ducts referred to in paragraph (b).


So now we understand that common property is everything that is not lot property, and we know that the lot is everything within the boundaries including internal walls, but does not include common infrastructure.

Simple right, well kind of I know, but legislation is never written in an easy to understand way.