A network wiring or connection fault
A single plate is completely dark while its neighbours are lit, or a whole branch of the house has dropped off while the rest is fine. The units on that run are not getting the network, and the most likely reason is the path to them.
The C-Bus cable or a connection on that run has failed. A dark unit surrounded by lit ones has lost its supply from the bus, and the cable between them is the thing in common.
What the C-Bus cable is
C-Bus runs on a data cable, commonly a pink Cat5-style cable in Australian installations, which loops from unit to unit carrying both the data and the low-voltage power. Because it is one shared path, a break in the middle of a run can take out everything beyond it while everything before it carries on normally. That branching pattern is usually the strongest clue about where the fault is.
This one is mains work. Stop here and book a licensed electrician. Switchboards, DIN-rail power supplies, dimmer and relay modules, and anything behind a faceplate are all live electrical work. There is no safe DIY version of this check.
Ask for an electrician with C-Bus accreditation if you can get one. A general electrician can safely isolate, test and replace hardware, but most cannot program the system afterwards, which is the half that makes your lights behave again.
What actually causes them
- Connections, not cable
- The overwhelming majority of bus faults are at terminations rather than in the middle of a healthy cable. Connectors work loose, get disturbed, or were marginal from the day they were made.
- Later building work
- A renovation, new downlights, solar, air conditioning, or a security system installed by someone who did not recognise the pink cable. This is the single most common cause in houses over ten years old.
- Water
- Outdoor runs, garage junctions and anything that has been near a roof leak. Water intrusion also explains faults that come and go with the weather.
- Rodents
- Roof spaces, and more common than people expect. The damage is usually obvious once someone is up there looking.
The dangerous version
Most wiring faults are an open circuit: a break, so everything past it goes quiet. The other kind is a short across the bus, and that one presents very differently. A short can pull the network voltage down so far that every plate in the house goes dark, which looks exactly like a failed power supply. It is one of the two reasons a competent person measures before ordering a power supply, and it is why a replacement supply that does not fix the problem should prompt a network test rather than a second replacement.
How it is found
By testing, and by the map of what works. An integrator uses what is dark and what is not to narrow the fault to a section, then tests that section. If you can tell them which plates are affected and roughly where they are in the house, you have done a real part of the diagnostic work before anyone arrives. Write the list down room by room: it is worth more than a description of the symptoms.
What repair involves
Locating the fault, then repairing or replacing the affected section or connection. Where a run has been damaged inside a wall, a new cable is sometimes pulled through an accessible path rather than opening the wall. Costs turn almost entirely on access rather than on parts, which is why nobody sensible quotes this one sight unseen.
One plate dark, or a whole branch gone?
Send the room-by-room list of what is affected. That map is what turns a wiring fault from an open-ended search into a targeted one, and it is the difference between a short visit and a long one.