On-site sewage systems overview
Who needs an on-site sewage system, how do they work, and why is it important to look after them?
Legal restrictions mean that on-site sewage systems, such as septic tanks, are generally only found in rural areas where a mains sewage system is not available.
Many New Zealand properties use septic tanks for on-site sewage treatment. But there are various other systems such as single waterless toilets, or 'aerobic' or 'secondary' treatment systems.
An on-site sewage system requires correct installation and regular maintenance to dispose of wastewater from your home safely, or it may pollute the environment, and put your family's and your neighbours’ health at risk.
Who needs an on-site sewage system?
The Building Code requires that if a connection to the mains sewer is provided, you must use it. However, if there is no connection to the mains sewer available then an on-site system is allowed.
Most people in New Zealand have access to a mains sewer, so an on-site system is not an option.
How do on-site sewage systems work?
Wastewater from your home is piped to the on-site sewage system which consists of one or more chambers. The chambers can be made of concrete, fibreglass, or plastic. In general, the system will:
- Separate solid waste from wastewater. Solids settle on the bottom of the chamber, and light waste such as fats and grease floats to the top.
- Pre-treat the wastewater by holding it until bacteria start to break down most of the biodegradable material and pathogens.
- Discharge the pre-treated wastewater through pipes into a soakage treatment area, where the sun and organisms in the soil combine to complete the treatment process.
See Choosing an on-site sewage system for more detailed information about tanks, chambers and soakage treatment areas.
Blackwater and greywater
Wastewater is the water you dispose of from your home and it comprises blackwater and greywater.
Blackwater is wastewater from the toilet and bidet - it must be treated through an on-site or mains sewage system. It can be a public health risk if not handled properly.
Greywater is wastewater from all other areas of the household such as the kitchen, laundry or bathroom (not including the toilet and bidet). It may also be discharged to the sewage system. In some areas, you may be allowed to re-use it on your garden or for flushing the toilet. However, there can be safety issues with doing this (see Re-using greywater).
Why on-site sewage systems matter
Properly set up and installed, on-site sewage systems treat wastewater to remove pathogens and chemical residues. However, if the system is not properly maintained or is failing for some reason, then it can become a serious health and environmental risk.
Household wastewater typically contains nitrates and phosphates, which can cause toxic algal bloom in waterways.
Household wastewater may contain:
Viral and bacterial infections are the most common causes of general sickness in New Zealand. The flu, colds, hepatitis and meningitis are caused by viruses. Viral gastroenteritis or hepatitis A are waterborne and so can be carried in human waste.
Bacteria can cause a number of infections including stomach upsets. Campylobacter and salmonella can be carried in human waste and can cause serious illness.
Protozoa are difficult to get rid of and cause diarrhoea, vomiting, nausea and stomach pain. Protozoa found in human waste include giardia and cryptosporidium.
Helminthes or worms can cause a number of unpleasant symptoms such as itching, fever or chills and muscle aches. The types of worms carried in human waste - hookworms, whipworms or roundworms are considered rare in New Zealand.
From Smarter Homes
- Choosing an on-site sewage system
- Maintaining your on-site sewage system
- Waterless toilets
- Easy ways to save water
- Reducing water flow
- Reusing greywater
- Collecting and using rainwater
- Managing stormwater
- Outdoor water use
From other sites
Your local or regional council will have information about on-site sewage systems in your area. Visit the Local Government website to check local authority boundaries and link to council websites.
The former Waitakere City Council’s website has a page about on-site sewage systems.
You can download On-site Wastewater Systems – Selecting a System for Your Property, an On-Site NZ booklet, and The Story of Your Septic Tank System, a joint Ministry for the Environment / NZ Water and Wastes Association booklet, from the Association's website.
You can buy copies of the New Zealand Standards relating to on-site sewage systems and waterless toilets from the Standards New Zealand website. See:
- AS/NZS 1547:2000 On-site domestic wastewater management
- AS/NZS 1546.1:2008 On-site domestic wastewater treatment units – Septic tanks.
- AS/NZS 1546.2:2008 On-site domestic wastewater treatment units - Waterless composting toilets.
- AS/NZS 1546.3:2008 On-site domestic wastewater treatment units – Aerated wastewater treatment systems.
The Level website has more detailed information on on-site wastewater treatment.