A blocked underground stormwater pipe is one of those problems that sneaks up on you — usually during a heavy downpour when your gutters are overflowing and water is pooling where it shouldn't. The good news is that many blockages can be cleared by a capable homeowner with the right tools and a methodical approach. However, some situations do require a licensed professional, so read on before you grab a hose.
Understanding Your Stormwater System
Your home's stormwater system collects rainwater from the roof via gutters and downpipes, then channels it underground through pipes to the street, a soakage pit, or a watercourse. These pipes are typically 100 mm diameter PVC and are separate from your sewage (wastewater) system. Mixing the two up is a common — and costly — mistake.
Before you start, confirm you're dealing with a stormwater pipe, not a wastewater drain. If the blockage is near a toilet, sink, or shower, stop and call a licensed drainlayer. Working on wastewater drains involves restricted work under the Plumbers Gasfitters & Drainlayers Board regulations.
Tools You'll Need
- Garden hose with a high-pressure nozzle or a water blaster
- Drain rods (flexible rods that screw together — available at Bunnings or Mitre 10)
- Plumber's drain snake (manual or electric)
- Rubber gloves, safety glasses, and old clothes
- A torch and a flat-head screwdriver (for lifting gully trap lids)
- Bucket and trowel
Step 1 — Locate the Blockage
Start at the downpipe where it enters the ground. Pour a bucket of water into the top of the downpipe and watch what happens. If water backs up immediately, the blockage is close to the entry point. If it drains slowly and backs up further down, the blockage is deeper in the line.
Check any inspection chambers (access points with removable lids) along the pipe run. Lift the lids carefully and look for standing water — this tells you the blockage is downstream of that point.
Step 2 — Clear Debris at the Entry Point
The most common culprit is a build-up of leaves, silt, and gutter debris at the junction where the downpipe meets the underground pipe. Use a gloved hand or trowel to scoop out any compacted material from the top of the pipe or gully trap. This alone often solves the problem.
Step 3 — Use a Garden Hose or Water Blaster
Insert a hose into the pipe as far as it will go and turn on the water at full pressure. The force can dislodge soft blockages like leaf matter and silt. A water blaster with a drain-cleaning nozzle (sometimes called a jetter nozzle) is significantly more effective and can be hired from most tool hire shops.
Work the hose back and forth while the water runs. Have someone watch the downstream end (e.g., the kerb channel or soakage pit outlet) to see when water starts flowing freely.
Step 4 — Use Drain Rods
If water pressure alone doesn't do the job, drain rods are your next option. Screw the sections together and feed them into the pipe, rotating clockwise as you push forward. Never rotate anti-clockwise — this can unscrew the rods and leave them stuck in the pipe.
Apply steady pressure to break through the blockage, then flush the pipe thoroughly with water once it clears.
Step 5 — Flush and Test
Once you believe the blockage is clear, run a sustained flow of water through the system — a full bucket poured quickly into the downpipe is a good test. Check all inspection chambers to confirm water is flowing freely to the outlet.
Preventing Future Blockages
- Clean your gutters at least twice a year (more often if you have overhanging trees)
- Fit leaf guards or downpipe strainers to catch debris before it enters the underground pipe
- Flush the underground pipes with a hose annually as routine maintenance
- Avoid planting trees with aggressive root systems near stormwater lines — roots are a leading cause of pipe damage
For guidance on maintaining your home's drainage to meet the NZ Building Code requirements (particularly Clause E1 — Surface Water), check with your local council.
When to Call a Professional
Some stormwater blockages are beyond DIY. Call a qualified drainlayer or drainage contractor if:
- Drain rods and jetting have failed — the blockage may be a collapsed pipe, tree root intrusion, or a displaced joint
- You suspect the pipe is cracked or broken (subsidence, sinkholes, or wet patches in the lawn are warning signs)
- The blocked pipe is shared with neighbours or connects to the council's network — this may require council notification and a licensed contractor
- You're unsure whether you're dealing with stormwater or wastewater — wastewater drain work is legally restricted to licensed practitioners registered with the PGDB
- The work requires excavation to access or replace pipe — this can affect foundations and may require a building consent; always check with your local council first
For peace of mind and a proper CCTV drain inspection, find a qualified local drainage professional on tradedirectory.co.nz.
Safety Note
Always wear rubber gloves and eye protection when working with drains. Even stormwater pipes can carry bacteria and contaminants, especially if there has been any cross-connection with wastewater. Wash your hands thoroughly after the job. If you're using an electric drain snake, keep it well away from standing water to avoid electric shock.
SiteWise
.png)
