When do you need a neutral wire?

Almost every smart switch listing carries one of two lines: 'neutral required' or 'no neutral needed'. The neutral in question is a specific conductor doing a specific job, and whether a device needs it comes down to whether it needs a supply of its own that the switched active alone can't give it.

What the neutral wire does

A light circuit needs two conductors to work. The active carries energy to the light, and the neutral carries it back. Break either one and the lamp goes out.

Australian homes run on a 230 V single-phase supply, and under the current rules three conductors arrive at each light point: active, neutral and earth. The earth is a safety conductor. It is bonded to the neutral at one designated point under the MEN system used here, and downstream of that the two stay separate. Earth exists to carry fault current so the protection disconnects, not to carry the light's return current.

So the lamp needs a neutral. A switch does not. A single-pole switch only interrupts the active: permanent active in, switched active out. Switching the neutral instead would turn the light off while leaving the lampholder live, which is why the wiring rules don't allow it on an ordinary switch.

What needs a neutral, and what doesn't

The rule is simple. Anything that needs a supply of its own, rather than just a path to interrupt, needs a neutral. That covers anything with electronics that keep working while the light is off, unless it is built to borrow its supply through the lamp instead.

That is the whole question. A neutral at the switch is not a mark of good wiring. It is either needed by what you are installing or it isn't.

Where the neutral sits in a lighting circuit

Australian training material describes loop at the light as the main method of wiring lights here. Supply active, neutral and earth all arrive at the light point, where they are joined and looped on to the next light, and only two conductors drop down the wall to the switch: permanent active down, switched active back up. Under that arrangement there is no neutral at the switch, and there was never meant to be.

The other arrangement is loop at the switch, where the supply and the outgoing cable meet in the switch box instead. A neutral is present there, though it is usually joined through in an insulated connector rather than landed on the switch mechanism. It is in the box. It is not necessarily on the switch.

Which one your place uses depends on when it was wired and what it was wired for, and it can differ room to room in the same house. Downlights, renovations and rewires all change the answer. Anyone telling you what is behind your wall plate without looking is guessing.

Why you can't check this yourself

An electrician has to tell you. Replacing a light switch is licensed electrical work everywhere in Australia. No state or territory has a homeowner or owner-builder exemption, and switching the breaker off doesn't create one. Western Australia's regulator tells homeowners never to remove switches or fittings even to paint around them, because live wires stay connected behind the plate. Turning the light off at the wall isn't isolation either: a correctly wired switch keeps permanent active present with the light off.

Colour won't settle it from a photo. Under the current wiring rules a neutral in fixed wiring is black or light blue, and red and black cable is still compliant and still manufactured, so red and black doesn't mean the house is old. White is a permitted active colour in Australia, and red and white twin active is the classic switch drop, so the American habit of reading white as neutral is actively misleading here. Black and light blue cores can also be legitimately re-identified as actives with sleeving at the terminations. An electrician tests. They don't go by colour.

Neutral and earth are not interchangeable

This one matters. Neutral and earth are bonded at the switchboard, which leads people to assume a spare earth will do as a return path. It won't, and using it that way is dangerous and illegal. Load current on the earth is load current on the exposed metalwork of your installation, and it creates parallel return paths that undermine fault protection and stop RCDs working the way they are meant to. Green and green/yellow conductors are earth and nothing else.

If there is no neutral at your switch

It rules out fewer products than you would think. There are two ways around it.

A no-neutral switch completes its own circuit by passing a small current through the lamp. It works, with conditions. Each model has a minimum load, from about 3 W up to 20 W depending on the switch, and low-wattage LEDs below that threshold tend to glow when off or flicker. A bypass capacitor fitted at the light fixes most of it. Shelly, for one, sells its own and requires it below 20 W. Each manufacturer publishes a lamp compatibility list worth reading first.

The other way is to put the smart part where the neutral already is. A relay module fits behind the light fitting or in the ceiling, where the neutral normally already is, and it uses your existing wall switch as its input. Nothing on the wall changes, the electronics get a proper supply instead of leaking through the lamp, and small LED loads behave. If you go that way, the Shelly picker asks about your wiring and names the module that fits it.

Either way the installation is an electrician's job, and it is usually a short one. If you are getting one in anyway, ask them to check for a neutral at the switch positions you care about while they are there.