Every winter, the same thing happens in a lot of Australian bathrooms. Hot shower, cold tiles, one small window that nobody opens because it's 8 degrees outside, and by August there's black spotting along the grout and a grey bloom creeping across the ceiling. Wiping it off treats the symptom. Getting the moisture out of the room in the first 20 minutes treats the cause.
A humidity sensor plus an automated exhaust fan is one of the highest-value, lowest-glamour smart home upgrades you can make. Here's how to do it properly.
Start Here: The Practical Version
Put a battery humidity sensor in the bathroom, roughly 1.5m off the floor, away from the shower spray and away from the fan itself. Run the exhaust fan automatically when humidity climbs sharply, and — this is the bit most people skip — keep running it for 15 to 20 minutes after the shower stops. That post-run is where the actual drying happens.
If your fan is hardwired (almost all Australian bathroom fans are), switching it automatically means a licensed electrician installs a smart relay or a smart switch behind the wall plate. That is not a DIY job in Australia. Renters and anyone not ready for electrical work can still get most of the benefit from a sensor plus a plug-in dehumidifier and a phone alert.
Why Australian Bathrooms Grow Mould in Winter
Mould needs moisture, a surface and time. Winter delivers all three at once.
Warm shower air holds a lot of water vapour. When that air touches a cold surface — a single-glazed window, an uninsulated external wall, a ceiling below an unheated roof space — it hits dew point and condenses. Melbourne, Hobart, Canberra and the Adelaide Hills see this badly because overnight surface temperatures drop low while indoor humidity stays high. Even in Brisbane and Perth, a closed-up bathroom in July holds moisture for hours.
The other factor is that a lot of older Australian housing stock has an exhaust fan that vents into the roof cavity rather than outside. That moves the problem rather than solving it, and it can cause insulation and framing issues over time. If you're unsure where yours discharges, that's worth checking before you automate anything — an electrician or handyperson can confirm whether the ducting terminates at an eave or roof vent. The current National Construction Code housing provisions call for bathroom exhaust to discharge to outdoor air. What applies to an existing home depends on when it was built and on any new work. Check with a builder, building certifier or your state building authority rather than assuming.
What Humidity Level Should You Actually Target?
Recommended indoor humidity ranges vary between sources. The National Asthma Council, for example, suggests keeping indoor relative humidity between 30% and 50% where possible, noting that mould can grow above 55% and that around 70% is ideal for it. But absolute thresholds make poor automation triggers in a bathroom, for two reasons.
First, ambient humidity swings by season. A Sydney bathroom might sit at 68% on a humid February morning without anyone showering, and 45% in a dry winter westerly. A fixed "run the fan above 65%" rule will run the fan all summer and never trigger in winter.
Second, showers create a spike, and the spike is what you want to catch. The better approach is a rate-of-change or delta trigger. Compare the bathroom sensor to a baseline (a hallway sensor, or the bathroom's own reading 15 minutes ago). Then run the fan when the difference exceeds around 8 to 12 percentage points. Then stop when the reading returns to within about 3 points of baseline, or after a maximum runtime of 30 to 40 minutes so a stuck reading doesn't run the fan all day.
Choosing a Humidity Sensor for a Wet Room
Bathrooms are hostile to electronics. Look for a sensor with a sensible reporting interval, decent battery life, and a display if you want to eyeball the number while you tune the automation.
TP-Link Tapo Smart Temperature & Humidity Monitor
Best for: Tapo households that already run (or will buy) a Tapo hub and want a readable room display plus alerts and automations for heaters, fans or humidifiers
What we like
- 2.7-inch e-ink display shows temperature, humidity, a comfort indicator, battery and signal, so the readings are useful even without opening the app (per TP-Link Australia)
- TP-Link quotes ±0.3°C and ±3% RH accuracy from a Swiss-made sensor, with data refreshed every two seconds
- Free cloud logging of up to two years of data, with free CSV export, according to TP-Link
- Runs on two AAA batteries (included); TP-Link claims up to two years of battery life based on its own lab testing
- Sits on a desk or mounts on a wall with the included 3M adhesive, per TP-Link Australia
Worth knowing
- Needs a Tapo hub (sold separately) for remote monitoring, automations, notifications and data export
- Uses TP-Link's 922 MHz sub-GHz radio rather than Zigbee or Wi-Fi, so it pairs with Tapo hubs rather than third-party hubs directly
- The display only shows temperatures between 0°C and 50°C, although TP-Link lists a wider operating range
- Retailer battery-life claims differ from TP-Link's (JB Hi-Fi's listing says up to a year)
Also stocked at: The Good GuysAmazon AUBing LeeOfficeworks
The Tapo unit is an easy starting point if you're already in the Tapo/TP-Link app. The E-ink style display makes it genuinely useful as a standalone hygrometer while you work out what your "normal" is. Specification-wise it's designed as an indoor monitor, not a wet-rated device — keep it off the shower wall.
On protocols: Wi-Fi humidity sensors are simple to set up but tend to report less often to save battery, and some are mains-powered, which is awkward in a bathroom. Zigbee sensors typically report faster and last longer on a coin cell, which matters when your trigger depends on catching a rise quickly.
Tuya Wifi Temperature Humidity Sensor Indoor Hygrometer Thermometer Detector Smart Life
Best for: Australian renters and homeowners wanting battery-powered indoor climate tracking via Tuya Smart Life, Google Home, or Alexa without needing a hub.
What we like
- Connects directly to 2.4 GHz Wi-Fi without requiring an additional smart-home hub.
- Integrated LCD screen provides at-a-glance readings for temperature, humidity, time, battery level, and Wi-Fi signal.
- Native support for the Tuya Smart and Smart Life apps, along with voice queries via Amazon Alexa and Google Home.
- Operates without any mandatory subscription fees for logging data or receiving threshold alerts.
Worth knowing
- Updates environmental data intermittently (often every 30 to 120 minutes) due to battery-saving deep-sleep cycles.
- Lacks native integration with Apple Home, Samsung SmartThings, and Matter.
- Display viewing angles can wash out and lose contrast when viewed from above eye level.
- Indoor only, with no weatherproofing or water resistance for outdoor placement.
Also stocked at: eBay AU
Budget Tuya/Smart Life sensors are usually cheap enough (prices vary by retailer and model) that you can put one in the bathroom and one in the hallway for a baseline comparison. If you run Home Assistant, check before buying whether the model exposes local control or is cloud-only — that determines whether your automation still works when the NBN drops out.
Placement rules that actually matter: mount roughly 1.5m high, at least 1m from the shower screen, not directly under the exhaust grille (it'll read dry air and cut the fan early), and not on the outside of the door. A sensor in the wrong spot produces an automation that feels random.
Switching the Fan: What Needs an Electrician
Replacing a bathroom fan switch with a smart switch or installing an in-wall relay behind the existing plate is fixed wiring work for a licensed electrician. Licensing is regulated state by state, so check with your state's electrical safety regulator. Energy Safe Victoria, for example, says electrical work isn't a DIY job, even for small jobs such as changing power points or light switches. Bathrooms also carry wiring-rule restrictions on where switches and other accessories can go; your electrician works to these, and your state regulator can answer questions about them. Please don't take the plate off and have a go.
What you can do: get a quote for a smart relay module fitted behind the existing fan switch, which keeps the manual switch working while adding automation. Ask the electrician to confirm the fan is rated for extended run times and that the duct discharges outside. If the fan is 20 years old and rattles, replacing it with a higher-flow model at the same time is usually cheaper than two separate call-outs.
Some newer bathroom fans ship with built-in humidity sensing and a run-on timer. Those solve the basic problem without any smart home involvement, but they're a black box — you can't tune the threshold, and you get no history to look at.
Renting, or Not Ready to Rewire?
You can still cut mould risk substantially without touching the wiring.
Run the existing fan manually but set a smart speaker or phone reminder tied to the sensor. When bathroom humidity crosses your threshold, get a notification saying "turn the fan on". A second one 20 minutes later can say "you can turn it off". Crude, but it changes behaviour.
Better, pair the sensor with a plug-in dehumidifier on a smart plug and let the humidity reading control the socket.
TP-Link Tapo P100 Mini Smart Wi-Fi Socket Plug
Best for: Renters and first-time smart home buyers who want simple app, Alexa or Google Assistant control of a lamp or fan without buying a hub
What we like
- Connects straight to home Wi-Fi, so no hub or bridge is needed (TP-Link AU, The Good Guys)
- Compact 76.5 x 43.5 x 42 mm body that TP-Link says is sized to avoid blocking the neighbouring socket
- Schedules, countdown timers and an Away Mode that switches devices on and off to simulate someone being home
- Works with Alexa, Google Assistant and Samsung SmartThings, per TP-Link Australia
- Listed as supported by Home Assistant's local TP-Link integration
Worth knowing
- 2.4 GHz Wi-Fi only; The Good Guys notes it does not support 5 GHz
- No energy monitoring listed, unlike TP-Link's P110-series plugs
- No Matter or Apple Home support mentioned on TP-Link's AU product page
- Home Assistant setup still asks for your TP-Link cloud account credentials
Also stocked at: JB Hi-FiBunningsOfficeworks
A compact 240V smart plug like the Tapo P100 lets you run the dehumidifier only when the bathroom is genuinely damp, rather than leaving it running and paying for it. Check the plug's rated load against the appliance before you commit — dehumidifiers and heaters draw far more than a lamp, and small smart plugs have real limits. TP-Link lists the AU Tapo P100 at a maximum load of 10A / 2300W and recommends keeping continuous loads at 80% or less of that; check the rating on whichever model you buy.
Also worth doing, and free: leave the shower screen open after use and squeegee the walls. Leave the bathroom door ajar so the room can equalise, and crack the window on dry winter days even for 10 minutes.
Tuning the Automation Over the First Fortnight
Don't set thresholds on day one. Let the sensor log for a week and look at the graph. You're looking for the resting baseline, the peak during a shower, and how long the room takes to come back down on its own. Then set your delta and your run-on timer from real numbers.
A few refinements worth adding once the basics work:
- Minimum run time. Stops the fan short-cycling on a quick hand wash.
- Maximum run time. A safety net if the sensor battery dies mid-reading.
- Night quietness. Some households drop the run-on to 10 minutes after 10pm, or skip the automation entirely if the fan is loud enough to wake people.
- A manual override. Make sure the wall switch still wins. Nobody wants to argue with their own house at 6am.
If you're running Home Assistant, log the sensor to a long-term database and check it again in September. The satisfying part is watching the winter peaks flatten out — and not finding anything new on the ceiling come spring.
Related guides
- Smart Home Automation for Beginners: Your First 5 Routines
- Smart Home Automation: How Scenes, Routines and Sensors Actually Work
- How to Set Up Vacation Mode on Your Smart Home
- Bedroom Automation: Sleep, Wake and Light Scheduling
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