Drain Backing Up When It Rains? What's Really Happening Under Your Yard

Published October 2026 · By Jack Schroeder, BPC Registered Plumber

Why Rain Makes a Drain Back Up

Your house has two separate drainage systems under the ground, and they're not supposed to meet. Stormwater takes rain from the roof, gutters, downpipes, paved areas and yard grates out to the street or a legal point of discharge. The sewer takes everything from toilets, showers, sinks and the laundry to the water authority's main. In dry weather a partly blocked pipe can still cope, because the flow is small. Tip a storm onto it and the same pipe has to move many times the water — and whatever is restricting it (roots, silt, a crushed section, a belly where the pipe has sagged) suddenly shows.

Rain backs a drain up in one of three ways:

• A blocked or undersized stormwater line. The most common one. Leaves, silt and tree roots build up in the stormwater pipe over years. The first heavy rain of the season finds the restriction and water comes back up the nearest grate, pools around the downpipe or runs across the driveway into the garage.
• Stormwater getting into the sewer. Sewer pipes aren't built for rain. If a downpipe has been wrongly connected into the sewer at some point, or an old earthenware sewer has cracked joints and the ground around it is saturated, rainwater pours in and the sewer surcharges. That's when you see the toilet bubbling, water rising in the shower or the overflow relief gully spilling — but only while it's raining.
• The street system is full. On a low block, or when the council pit on the street is itself blocked or overwhelmed, water can't leave your property fast enough even if your own pipes are clear. This is the one case where the fix isn't on your side of the fence — but you still need to prove it, and a camera does that.

Which One Is It? Read the Symptoms

Where the water shows up tells you which system is in trouble. Use this before you ring anyone — it'll shorten the conversation.

If you've got symptoms from both lists at once, stormwater is almost certainly getting into the sewer somewhere on the property. That's worth fixing properly rather than clearing every winter, because every litre of rain in the sewer is a litre that can come up through your bathroom floor.

What to Do Right Now

Is It Council's Problem, the Water Authority's, or Mine?

This is the question that stops people acting, so here's the short version for Melbourne's east.

• Stormwater on your property — pipes, pits, grates and downpipes up to the point where it leaves your block — is the property owner's responsibility. Councils (Knox, Maroondah, Yarra Ranges, Whitehorse and the rest) maintain the pits and pipes in the street and the legal point of discharge you connect to, and Melbourne Water looks after the larger main drains and creeks. If the council pit at your kerb is full of leaves, report it to council; if the water is backing up on your side before it ever reaches the street, that's yours.
• Sewer — Yarra Valley Water (most of Knox, Maroondah, the Ranges and the Yarra Valley) and South East Water (Rowville, Wantirna, Knoxfield, Upwey and south) own the main in the street and the connection up to a defined point near your boundary; the house drain from there to your fixtures is yours. Yarra Valley Water's 2026 repair-responsibilities guidance also notes that if your inspection shaft is buried, a private plumber has to raise it before they'll attend. In practice you need your own plumber to clear and camera the private drain first; if the footage shows the fault is on the authority's side, the authority takes it from there and you may be able to claim some costs back.
• Renters: a drain that backs up sewage, or a toilet that won't flush, is an urgent repair under Victoria's Residential Tenancies Act — contact your property manager straight away and keep the video. Our who-is-responsible guide covers the tenant / landlord / water-authority split in detail.

Whoever ends up paying, the CCTV footage is the thing that settles it. That's why we run the camera on every rain-related job rather than just clearing the pipe and leaving.

What the Fix Actually Looks Like

Clearing a drain that only fails in rain is a two-part job: get it flowing, then find out why it stopped.

1. Jet it. A 5000 PSI water jetter cuts roots and scours silt and leaf sludge off the pipe walls — the full bore, not a hole through the middle the way an eel does. Stormwater lines in particular are usually half-full of fine silt that an eel just stirs around.
2. Camera it. With the line clear, the camera goes down and we see what caused it: root intrusion at a joint, a sagged section holding water and silt, a crushed pipe under a driveway, or — on the sewer — a downpipe that someone connected into it decades ago.
3. Fix the cause. Roots in otherwise sound pipe: jet on a schedule or reline the section so they can't get back in. Crushed or collapsed pipe: dig and replace that section. Wrong connection: disconnect the downpipe and run it to stormwater where it belongs. Silted pit with no outlet: clean it and, if needed, give it a proper outlet or soakage.

If you'd rather know where you stand before the next storm hits, the stormwater drain health check is exactly that — camera through the stormwater, silt and root report, no clearing unless you want it.

What It Costs

Fixed price before work starts, no call-out fee in business hours. As a guide, clearing a single blocked grate, pit or downpipe line starts from around $300; a main-line jet with the camera through afterwards is typically $500–$750. Relining or replacing a damaged section is quoted separately once we've seen the footage — you'll have the video and a written report before deciding anything. The full breakdown is in our blocked drain cost guide.

This Season in Melbourne's East

The early-October 2026 storms gave a lot of homes their first look at this problem for the year — Melbourne's wettest day since April 2024, with flash flooding across the suburbs. The Bureau of Meteorology has an El Niño in place for this summer, which usually means fewer rain days but doesn't rule out the short, intense downpours that overwhelm a half-blocked stormwater line in minutes. Drier ground also sends tree roots further in search of moisture, and a leaking sewer joint is exactly where they find it. So the pattern to expect is long dry spells, then a storm that finds every weak pipe at once.

The places we see it most: the steep blocks of Kalorama, Olinda and Belgrave, where stormwater moves fast and carries leaf litter straight into the pits; the flat clay of Rowville and Wantirna, where water sits and silt settles; and the older earthenware sewers of Boronia, Bayswater and Ringwood, where roots in the joints let rain in.

Frequently Asked Questions