Septic tanks

It is estimated that around 14,600 Otago properties, or 38,000 people (around one in five Otago residents) are serviced by septic tanks.

The Otago Water Plan includes permitted activity rules covering small-scale discharges from septic tanks and long drops. These make provision for the discharge of human sewage , provided certain conditions are met. These conditions vary depending on whether the discharge was put in before or after 28 February 1998.

Note: The approval of particular technologies for the on-site treatment of human sewage will usually require the approval of your city or district council. The Otago Water Plan deals only with the effect of the discharge on the environment.

Rules for septic tank

Rules for septic tanks are covered in section 12A ( Page 12-31) of the Otago Water Plan.

Septic Tank Maintenance

The basic operation of a septic tank relies on its ability to retain wastewater for long enough to enable solids, fats, and oils to settle.

Typical septic tanks operate by allowing solid material to settle out to the bottom of the tank as sludge, and lighter fats and oils to float to the top. Septic tanks also rely on the right kind of flora (‘good bacteria’) to breakdown some of the contents.

Although having some sludge present in the bottom of your septic tank is necessary to support the good bacteria, having too much sludge in your septic tank will reduce the amount of time wastewater can be held for, resulting in insufficient wastewater treatment, potentially creating an environmental problem.

On average a septic tank should be pumped clear of sludge every three to five years, however, some may require pumping out on an annual basis. How regularly your septic tank requires pumping out will depend on what steps you take to conserve water and reduce sludge build-up.


Do scrape your dishes clean of food and fats before washing
Do remove sand and soil from your clothes before washing them
Do install water saving devices
Do use showers instead of baths
Do use biodegradable soaps and washing powder
Do use detergents and cleaners compatible with your system
Do protect your septic tanks and disposal fields from vehicle access to avoid cracking the pipes and tank.
Do fix leaky taps
Do try and avoid doing more than one full load of laundry a day.


Don’t use a waste disposal unit
Don’t flush sanitary products, disposable nappies etc
Don’t flush expired medicines
Don’t use washing machines or dishwashers unless you have full loads
Don’t overuse strong bleaches and disinfectants
Don’t put paints, weed killers and other chemicals down the drain
Don’t put oils, fats, or greases down your drain
Don’t use septic tank cleaning chemicals unless specified by the manufacturer
Don’t allow storm-water to enter the disposal field
Don’t put large volumes of water through your system (e.g. from spa pools)
Don’t plant deep rooting trees or shrubs over the disposal field and pipes.

Septic tank failure

Septic tank failure occurs when your septic tank no longer provides sufficient treatment for its contents. This happens when the good bacteria die off, your septic tank is damaged, or your disposal field is damaged.

When a septic tank fails it results in untreated or partially treated wastewater being released into the environment. Failing septic tanks can contaminate streams and wells, make people sick and are unpleasant to be around.

Your septic tank may be failing if you have:

  • Waste-water ponding at the surface
  • Smell of sewage near the septic tank or disposal field
  • Slow draining sinks and toilets
  • Sinks backing up when the toilet is flushed
  • Black and slimy areas around your tank or disposal field
  • Evidence of discharges around vents or gully traps.

Septic tanks may fail for any of the following reasons:

  • Build-up of sludge
  • Tree roots extending into the disposal system
  • Heavy stock or vehicles fracturing the system
  • Poor location of the system
  • Good bacteria being killed off.

If you find evidence that your septic tank is failing then you may need to arrange to have it pumped out as well as getting a suitably qualified plumber or drain-layer to inspect your system.

Groundwater contamination risk, septic tank density and distribution within Otago

This report above examines some of the problems associated with septic tanks and their management under the current Regional Plan: Water for Otago (RPW) ‘permitted activity’ rules, and suggests some ways in which they might be managed in the future. Modelling of the number of septic tanks and the susceptibility of groundwater to septic tank leachate contamination was undertaken to facilitate ‘permitted activity’ management within the Otago Region.


If you have any more questions, please get in contact with us at

Generic cutaway of a typical septic tank and disposal field

Generic cutaway of a typical septic tank and disposal field

Failing disposal field

Failing disposal field

Failing disposal field

Failing disposal field

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