Four months, give or take, is roughly the window you get. Anything before late May and the creeks in the Perth Hills are usually bone dry, and anything after October you're looking at a trickle over mossy granite that photographs well but isn't really a waterfall so much as a damp rock. I've been catching these falls on and off for years now and the lesson never changes: check the rain before you check the map.
Perth's hills country sits on shallow soils over granite, which means the creeks that feed places like Lesmurdie Falls and the falls inside John Forrest National Park don't hold water the way a proper river system does. They're flash systems. A good winter front rolls through, the gullies fill, and for a few weeks you get genuine white water tumbling over the scarp. Then the sun comes back out for a fortnight in September and it's over for another year. The WA Dept. of Biodiversity, Conservation and Attractions manages most of this country and their seasonal closure notices are honestly the most useful tool going, more useful than any Instagram post claiming a spot is "flowing beautifully" from three weeks ago.
Why Perth Hills waterfalls only run after real rain
The short version is geology. The Darling Scarp is old, hard granite with a thin skin of soil on top, so rain doesn't soak in and slowly release the way it might in sandier country closer to the coast. It runs off fast, fills the gullies, and drains away just as fast once the fronts stop coming through. That's why you'll hear locals talk about "a good winter" versus a lean one — 2021 was a good one, plenty of flow well into August, whereas some years the falls barely wet their feet before drying up by early September.
What this means practically is that a dry spell of even ten days in June or July can knock the flow right back, and but a single decent front through the hills can bring it roaring back within a day or two. I've turned up to a trickle on a Tuesday and found a proper cascade by the following weekend, purely because 20mm fell overnight. There's no substitute for checking recent rainfall before you drive out, rather than relying on old photos.
Lesmurdie Falls and the crowd that comes with it
Lesmurdie Falls is the one everyone posts. Fair enough, it's close to the city, the lookout is a short stroll from the car park, and after a wet week the drop is genuinely impressive. But I'll say this plainly: the social media version oversells it. Most weekends in peak flow you're sharing the viewing platform with forty other people trying to get the same angle, and the falls themselves run for maybe six or seven weeks a year at anything like their best. If you've seen the photos and expect that every visit, you'll be disappointed more often than not.
I'd rather point people toward the quieter falls further into the hills, particularly the ones inside John Forrest National Park, National Park Falls and Hovea Falls. Same catchment, same seasonal rules, a fraction of the foot traffic. They don't photograph quite as dramatically from a single postcard angle, but standing there on a weekday with nobody else around is worth more to me than a better photo with a queue behind it. That's the mildly unpopular opinion I'll stand by: the famous one isn't always the best one.
Walking tracks that put you near the flow
You don't need to walk straight up to a waterfall to get a feel for how wet the hills are running. Some of the best indicators are the creek crossings on established walks well away from the falls themselves. Smith Road crosses several of these seasonal drainage lines and gives you a decent read on how much water is moving through the catchment before you commit to the drive further into the falls country. Helena Valley Road does something similar lower down in the Helena catchment, and I've used both as a rough gauge before deciding whether a Saturday morning is worth the fuel.
If you're riding rather than walking, the day on the Munda Biddi Trail's Perth Hills section passes through similar granite country and gives you a good sense of how fast these gullies fill and empty across a season. I measure most hills walking against the Bibbulmun Track Foundation's grading system out of habit, it's the yardstick I know best, and by that scale most of the falls walks sit at an easy-to-moderate grade. Nothing here compares to a full day on the Bibbulmun's harder southern sections, but the footing gets greasy on wet granite and I've gone over once on a slick rock shelf near Hovea, so don't treat easy as meaning careless.
Base camp and getting your bearings
For anyone making a weekend of it, the Perth Hills Discovery Centre and Campground in Mundaring is a sensible base. It sits close enough to the main falls clusters that you're not burning half a tank driving between them, and the staff there generally have a good read on which creeks are running that week, which beats guessing off old trip reports. If you'd rather not manage logistics yourself, the Perth Hills Explorer Private Day Tour covers several of the falls and lookout points in a single day with someone else doing the driving, which on a wet winter morning with fogged-up windscreens is not nothing.
What I carried and what I'd change
Two water bottles, wool socks rather than the usual cotton ones because wet granite means wet feet eventually no matter how careful you are, and gaiters I didn't end up needing but was glad to have in the pack. The mud around the creek crossings after a big rain event can be ankle deep in places, particularly on the approach tracks that aren't as well maintained as the main falls trails. I skipped the gaiters on one trip to Hovea and regretted it by the second crossing, socks soaked through, boots squelching for the rest of the walk. Small thing, but it changes how much you enjoy the last hour.
Timing it right
My rule of thumb is simple. Check the week's rainfall, not the season's average, because a wet June followed by a dry July will leave the falls disappointing by August even though the calendar says it's still winter. Weekday mornings after a midweek front tend to be the sweet spot, good flow and nobody else on the viewing platform. Weekend afternoons after a run of sunny days are the worst combination, crowded and underwhelming at the same time.
If you're planning around a specific weekend, give the DBCA site a look a day or two out rather than relying on what someone posted a fortnight ago. Waterfalls in this part of the world don't wait around for your schedule, and by the time the jacarandas are out in Perth's suburbs, most of these creeks have already gone quiet for another year.