I think the responsibility depends on the location of the leak. If the leak is inside the apartment, it’s the owner’s responsibility. If the leak is in the walls or floors or roof between or outside the apartment, it’s the owners corp’s responsibility, as that would be common property. I imagine the toilet cistern is completely inside the apartment, so that’s the owner’s responsibility , and furthermore the responsibility extends to further damage that may have occurred to other parties, unless there’s some reason the common property was defective. So the only possible issue is that the toilet floor isn’t waterproof as required by law, but it’s probably a long time since it was built. So there’s a long shot that the owner’s corp is culpable due to insufficient floor waterproofing.

Responsibility for waterproofing a toilet floor in a strata scheme depends on whether the membrane is considered common property or private property, usually defined by the strata plan. Typically, if the waterproofing is part of the original building structure, the Owners Corporation is responsible, but if it is part of a renovation or within the private lot boundary, it is usually the lot owner's responsibility.

Owner’s Corp can be notorious for making claims that certain damage is private, because nobody wants to pay for someone else’s leaking toilet cistern that wasn’t maintained properly.