When the drain is not entirely yours
In a stand-alone house, the drainage on the property is generally the owner’s to sort out, and the decision is simple: something is blocked, get someone to clear it. In an apartment, a duplex or anything under body corporate management, the pipework divides. Some of it serves your lot alone. Some of it is common property serving several lots or the whole building.
That division matters because it determines who can authorise work and who pays for it. Arranging a plumber for a blockage that turns out to be in common property can leave you holding an invoice for something the body corporate would have handled. Waiting for the body corporate to act on a blockage inside your own lot wastes days on a problem that was always yours.
The useful signal is the same one that separates a fixture blockage from a main-line blockage anywhere else: how many places are affected. If it is only your kitchen sink, the pipework in question is almost certainly serving your lot. If your neighbours are having the same problem, or waste water is appearing in a shared area, stairwell or car park, that is a strong indication the shared line is involved and the building manager or body corporate should be told before anything else happens.
It is worth finding out, before you have a problem, who your building manager is and how after-hours issues are meant to be reported. On the day it matters, that is the difference between one phone call and an afternoon of them.