On-site wastewater systems treat domestic wastewater and return it to the environment inside the property boundaries.  

Around 14,600 Otago properties are serviced by septic tanks. This equates to 38,000 people, or one in five Otago residents. We provide guidance on the use of on-site wastewater systems and how to reduce the risk to the environment.  

Permitted activity rules in Otago’s Water Plan (Regional Plan: Water) cover small-scale discharges from on-site wastewater systems, including septic tanks and long drops or pit toilets. These provide for the discharge of human wastewater, provided certain conditions are met. The conditions vary depending on whether the discharge was put in before or after 28 February 1998. Please review the most current Regional Plan for up-to-date information. 

Note: Your city or district council will usually need to approve technologies for the on-site treatment of human sewage. The Water Plan deals only with the effect of the discharge on the environment.

Types of wastewater systems

Primary systems

A primary system separates solids from the liquid waste. A septic tank with a disposal field is the simplest and most basic wastewater treatment system – the waste receives minimal treatment. The liquid component goes to a disposal field while the solids remain in the tank and get removed periodically by the system owner. Some pathogens and nutrients remain in the discharge when it is discharged to land. These units tend to be older.

Typical septic tank and disposal field system. Source: NIWA

Secondary or tertiary systems

Secondary and tertiary systems have multiple treatment tanks and a disposal field. They often use electric pumps and have multiple chambers for increased filtration.

Other systems

Other systems include composting toilets and worm farms, which have their own maintenance requirements.
The NZ Standard ‘AS/NZS 1547:2012 On-site domestic wastewater management’ sets out the requirements for treatment units and their land application systems to manage on-site wastewater so that public health and the environment are protected. 

Rules for on-site wastewater systems, including septic tanks

Rules for on-site wastewater systems, including septic tanks, are covered in section 12.A of the Otago Water Plan.
•    Rule 12.A.1.3 applies to systems installed before 28 February 1998.
•    Rule 12.A.1.4 applies to systems installed after 28 February 1998.
•    Rules 12.A.1.1 and 12.A.1.2 relate to long-drop toilets. 

You will need a resource consent if you cannot comply with the permitted activity standards – for example, your new system will discharge more than 2,000 litres per day or effluent is going onto a neighbouring property. 

Septic tanks

Septic system maintenance

Septic tanks retain wastewater for long enough to settle solids to the bottom of the tank as sludge, while fats and oils float to the top. The liquid is then slowly discharged to land. Septic tanks rely on the right kind of flora (‘good bacteria’) to break down some of the contents.

Although some sludge is needed in the bottom of the tank to support the good bacteria, too much will reduce the amount of time the system can hold wastewater for. This means the wastewater cannot be treated properly, which could create an environmental problem. A septic tank should be pumped clear of sludge every three to five years (on average), though some may need annual pumping out. The timeframe depends on what you do to conserve water and reduce sludge build-up. 

Take care of your system. Property owners are responsible for maintaining their on-site wastewater system and knowing how it works. Taking care of your system is not complicated and does not need to be costly. However not taking care of your system can damage the environment and could end up being very expensive, especially if you have to replace the system  . Discharges containing contaminants from inadequate on-site wastewater systems could end up in nearby domestic drinking water supplies and make people sick. Poorly treated discharge can pollute nearby waterways, including popular bathing beaches. It is therefore very important to maintain your wastewater treatment system well. 

Septic systems have a lifespan of around 20 years. Regular maintenance and care of your system will keep it working effectively for many years to come.


Reduce your water consumption and think about what goes down the drain
Get familiar with your septic system as good maintenance begins with understanding what type of system you have, how it works and where it’s located.
Regularly inspect and maintain your septic system.
Protect your septic tanks and disposal fields from vehicle access to avoid cracking the pipes and tank.
Scrape your dishes clean of food and fats before washing them.
Remove sand and soil from your clothes before washing them.
Install water-saving devices.
Fix leaky taps.
Take showers instead of baths.
Use detergents and cleaners compatible with your system.
Use biodegradable soaps and washing powder. 
Use alternative household cleaning products such as baking soda (for cleaning surfaces), white vinegar (for cleaning tile grout with a toothbrush) or lemon juice (one cup of juice in half a bucket of water instead of bleach).
Avoid doing more than one full load of laundry a day.
Talk with a service provider about improving the system.


Don’t use a waste disposal unit.
Don’t flush sanitary products, disposable nappies, etc.
Don’t flush unwanted medicines.
Don’t use washing machines or dishwashers unless you have full loads.
Don’t overuse strong bleaches, chlorine and disinfectants.
Don’t put paints, weedkillers and other chemicals down the drain.
Don’t put oils, fats or greases down your drain.
Don’t use septic tank cleaning chemicals  unless they are the ones specified by your system’s manufacturer.
Don’t put coffee grounds down the sink. 
Don’t allow stormwater into 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.


Signs of a failing disposal field can include wet, soggy areas in or around the drain field. There may also be odours near the tank or disposal field

Septic system failure

Septic system failure occurs when your septic tank is no longer able to treat its contents. This happens when the good bacteria die off or your septic tank or disposal field is aged, damaged or too small for your family size or household use.

When a septic tank fails, untreated or partially treated wastewater is released into the environment. Offensive odours, effluent seepage and other forms of environmental pollution caused by faulty or failing on-site wastewater systems must be stopped as soon as possible. Failing septic tanks can contaminate streams and wells, make people sick and are unpleasant to be around.

Septic systems may fail due to:

  • 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
  • being too small or poorly designed for the current load

Your septic system may be failing or need maintenance if you notice:

  • broken tank lids
  • pipe blockages
  • puddles of (often smelly) standing water in or around your disposal field or where your septic tank is buried
  • smell of wastewater near the septic tank or disposal field
  • slow draining toilets and showers is most often the first sign of a failing system
  • scum and sludge build-up in your septic tank
  • abnormally high water level in the septic tank
  • 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
  • bright green, spongy lush grass where your disposal field/septic tank is located even during dry weather
  • algal blooms in nearby ponds or lakes
  • high levels of nutrients or bacteria in water bores or wells

If your septic tank fails, you may need to have it pumped out and get a suitably qualified plumber or drain-layer to inspect your system.

Dead areas or stripes of brown/yellow grass above your disposal field during dry or hot weather might not be a bad thing as it could indicate that your disposal field is absorbing the wastewater and filtering it into the soil. The grass should recover when the cooler, wetter seasons arrive.

However, if it doesn’t, or if yellow or dead grass develops directly over the buried septic tank or pipelines then this could indicate a nutrient or chemical overload in the soil and an inspection may be needed. 


Disposal field is absorbing the wastewater


Reporting pollution

If you notice problems with an on-site wastewater system or pollution in your community, please contact our 24/7 pollution hotline on 800 800 033 or email pollution@orc.govt.nz.