Digging around an old concrete septic tank is a job that demands respect — for what's underground, what's inside the tank, and the structural condition of ageing concrete. Done carefully and methodically, you can safely expose a tank for inspection, pumping, or replacement. Rush it or cut corners, however, and you risk tank collapse, toxic gas exposure, or hitting buried services.
Before You Break Ground: Locate All Underground Services
The single most important step before any excavation is identifying what else is buried in the area. Underground power cables, water mains, gas lines, and stormwater pipes are all common around NZ properties — and hitting them can be fatal or extremely costly.
- Contact Dial Before You Dig (0800 248 344) to request a service location report for your site.
- Hire a cable and pipe locator (many hire companies offer these) or engage a professional locating service.
- Check your council's property file and any existing drainage plans for the location of inlet and outlet pipes connected to the tank.
- Mark all located services clearly with spray paint or flags before any machine or hand digging begins.
MBIE Building Performance has guidance on working near existing infrastructure, and your local council may hold drainage records that show the tank's position and connected pipework.
Assess the Condition of the Concrete Tank
Old concrete septic tanks — particularly those installed before the 1990s — can be in poor structural condition. Concrete degrades over time due to hydrogen sulphide gas produced inside the tank, ground movement, and tree root intrusion. Before and during excavation:
- Do not assume the lid is sound. Old lids can be paper-thin and may collapse under foot or machine pressure.
- Look for cracking, spalling, or subsidence in the ground above the tank — these are warning signs.
- If the tank is deeper than 1.5 metres to the base, you are working in a confined space situation and additional safety measures apply (see below).
- Avoid parking heavy machinery directly over the tank — the weight can cause sudden collapse.
Safe Excavation Techniques
Hand Digging vs. Machine Digging
For the area immediately around and above the tank, hand digging is strongly recommended once you are within 300–500 mm of the known tank position. Use a spade or mattock carefully, and probe with a steel rod to confirm the tank's edges before committing to deeper digging.
A mini excavator can be used to remove bulk soil away from the tank, but the operator must be experienced and cautious. Bucket teeth can punch through a deteriorated concrete lid with very little force.
Shoring and Batter Slopes
If you are digging a trench or pit deeper than 1.5 metres, WorkSafe NZ requires that the excavation is either:
- Battered back to a safe angle (typically 45° or less depending on soil type), or
- Shored with timber, steel trench boxes, or hydraulic shoring.
Never enter an unshored excavation deeper than 1.5 metres. Soil collapse can happen without warning and is frequently fatal.
Working Around the Inlet and Outlet Pipes
Septic tanks have at least one inlet pipe (from the house) and one outlet pipe (to a soakage field or secondary treatment). These pipes are often brittle — especially older clay or concrete pipes. Dig carefully by hand around them and avoid levering or twisting them, as a broken inlet pipe can cause sewage to back up into the house.
Toxic Gas and Confined Space Hazards
This is the most serious safety risk associated with septic tanks. The decomposition of sewage produces hydrogen sulphide (H₂S) and methane, both of which can be present at dangerous concentrations even when the tank lid is simply cracked or lifted.
- Never lean over an open tank or put your head near the opening.
- Do not enter the tank under any circumstances unless you are a trained confined space worker with appropriate gas detection equipment, breathing apparatus, and a standby person present — this is specialist work.
- Keep children and pets well away from the work area.
- Work in well-ventilated conditions and position yourself upwind of the tank opening.
Refer to WorkSafe NZ for confined space entry requirements and health and safety obligations.
Drainage and Consent Considerations
In New Zealand, septic tank systems are regulated under the NZ Building Code (in particular Clause G13 — Foul Water) and must comply with your local council's requirements. If you are replacing or significantly altering a septic tank, you will almost certainly need a building consent. Even for inspection or pump-out access, check with your council first.
Drainlaying work — including connecting or disconnecting pipes to a septic tank — is restricted work under the Plumbers, Gasfitters and Drainlayers Act 2006. It must be carried out by, or under the supervision of, a licensed drainlayer. Check licensing at the Plumbers Gasfitters & Drainlayers Board.
Personal Protective Equipment (PPE)
Always wear the following when working around a septic tank:
- Waterproof gloves (heavy-duty rubber or nitrile)
- Safety boots with steel toecaps
- Eye protection when digging or when near the open tank
- High-visibility vest if working near a road or driveway
- Wash hands thoroughly before eating or drinking, and shower after the job
When to Call a Professional
Call a licensed earthmover, drainlayer, or septic tank specialist if:
- The tank is deeper than 1.5 metres and you lack shoring equipment or experience.
- The concrete appears severely deteriorated or the lid has partially collapsed.
- You need to disconnect or reconnect any drainage pipework.
- You are replacing the tank entirely — this requires a building consent and licensed drainlaying work.
- You are unsure of the tank's exact location or condition.
A qualified earthmover with septic tank experience will have the right machinery, shoring gear, and knowledge to do this safely. You can find experienced local earthmoving contractors on tradedirectory.co.nz.
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