On-site sewage systems

Last updated: 9 August 2023

onsite sewage systems

Who needs an on-site sewage system, how do they work, and why is it important to look after them?

An on-site sewage system can be a good option if connection to the main system is not possible. They must be designed and sized correctly to work well. An undersized, poorly designed or under-maintained system may not adequately cope with your household's wastewater. It is the owner's responsibility to ensure on-site sewage systems are maintained and operating correctly to avoid endangering human health and the environment.

Any system needs to be properly installed and maintained, as well as big enough and efficient enough to treat all of the wastewater from your household – both now and in the future.

REQUIREMENTS - On-site sewage systems

If your property has a mains sewer connection, you're legally required to use it but the local authority can provide a waiver allowing on-site wastewater treatment to be installed.

You are permitted to install a greywater system, even if you are connected to the mains sewer. Check out the difference between greywater and blackwater.

Foul water has more information about Building Code requirements.

Legal restrictions mean that on-site sewage systems, such as septic tanks, are generally only found in rural or semi-rural areas where mains sewage is not available.

Septic tanks are the most commonly used for on-site sewage treatment. There are various other systems as well, including single waterless toilets, or 'aerobic' or 'secondary' treatment systems.

An on-site sewage system can deal with all your greywater and blackwater. The systems require correct design, installation and regular maintenance to dispose of wastewater from your home safely. Poor design, installation or maintenance may result in the system polluting the environment, and putting your family's and your neighbours’ health at risk.

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. Modern aerated and advanced wastewater treatment systems (AWTS) apply a number of treatment steps including:

  1. 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.
  2. Effluent passes through filter to aeration chamber. Pumped in air creates turbulence and aerates effluent. The chamber may incorporate a bioreactor to give additional bacterial treatment.
  3. Effluent passes through second, finer filter to clarification chamber. Fine sludge particles settle and are pumped back to the first chamber.
  4. Discharge of the treated wastewater through pipes into a soakage treatment area or tank where, depending on the system's treatment level, it can be used for watering the garden, toilet flushing and other applications.

Choosing an on-site sewage system has 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 containing human waste such as from the toilet and bidet. Greywater sourced from dishwashers and the kitchen cannot be recycled because it contains fats, detergents and cleaning agents, and can contain bacteria such as campylobacter and salmonella. Blackwater and kitchen water must be treated through an on-site or mains sewage system. It can be a public health risk if not handled properly.

Greywater from all other areas of the household such as the laundry or bathroom (not including the toilet and bidet) may also be discharged to the sewage system. In some areas, you may be allowed to reuse laundry and/or bathroom water on your garden or for flushing the toilet.

Reusing greywater explains related safety issues.

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.

Plant nutrients

Household wastewater typically contains nitrates and phosphates, which can cause toxic algal bloom in waterways.


Household wastewater may contain:

  • viruses
  • bacteria
  • protozoa
  • helminths.

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 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. They are found in human waste and 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 are considered rare in New Zealand. They include hookworms, whipworms or roundworms.

Choosing an on-site sewage system

What types of system?

On-site sewage systems can either break waste down using oxygen (aerobically) or without oxygen (anaerobically).

Traditional septic tanks generally have only one chamber and process wastewater anaerobically (this is sometimes referred to as 'primary treatment').

Other, more advanced systems have two or more chambers and use both anaerobic and aerobic process to break down the waste. (The aerobic part of this process is sometimes referred to as 'secondary treatment'.)

By providing secondary treatment, the more advanced systems produce effluent that is safer for human health and the environment.

Choosing the right system for your property can be complex. There are many systems available, with different numbers of chambers and different treatment processes.

But, as general rule, you're better off with a system that:

  • has at least two chambers, and
  • provides both primary and secondary treatment.

Buying a cheaper system that provides only primary, anaerobic treatment could turn out to be a false economy – you could end up having more problems with it and spending more on maintenance, as well as facing potentially greater health risks.

Many councils require new systems to have at least two chambers which is better than one.

A multi-chamber system can be added to an existing septic tank to improve its effectiveness. You can also add an outlet filter to improve the efficiency of a septic tank.

Maintenance, running costs and other considerations

The maintenance and running costs of the system are an important factor in making a decision about what type of system to get. All on-site wastewater systems require regular maintenance and cleaning – you will usually need a specialist to do this. Many councils now require proof of maintenance as part of consent conditions for on-site wastewater systems.

Some types of systems require electricity to work, and that may be a consideration for households in remote areas.

How big should your system be?

Your on-site sewage system needs to be big enough to deal with all of your house's wastewater – an average New Zealander uses 160–250 litres of water a day. For a three-bedroom house, you'll need a tank with at least 3000 litres' capacity. Some councils require at least this size.

Note that your system needs to be big enough to meet future needs as well as current ones. If you have a four-bedroom home but only three people live there, you'll need to install a system that's big enough for at least five people.

If your system isn't big enough, the consequences can be serious. You may be responsible for ground or surface water containing harmful bacteria (such as campylobacter and salmonella), protozoa (such as giardia and cryptosporidium), and other contaminants.

The soakage treatment area

Treated effluent from your septic tank is discharged to a soakage treatment area, where any remaining pathogens are removed before the effluent reaches nearby groundwater or waterways.

There are several types of soakage treatment area. Most use pipes with small holes to distribute effluent slowly either on or immediately under the surface of the soil.

Effluent gets to the soakage area either using gravity or a pump. Gravity won't always disperse the effluent evenly. This can cause clogging. With a pump, clogging shouldn't be a problem – but an unreliable power supply might be.

Native grasses, sedges, rushes and other moisture-loving plants will grow in your soakage area and will enhance the soakage effect. Check with your regional council for more information on what's suitable to plant in your area. Don't grow deep-rooting trees over the soakage treatment area.

The soakage treatment area should be:

  • large enough to cope with the amount of wastewater your household produces
  • as dry as possible – pathogens survive better in waterlogged soil
  • shallow – this allows plants to absorb nitrates and organisms in the soil and the heat of the sun to act on pathogens to remove them
  • away from waterways, flood-prone areas and areas of stormwater runoff.

Soil type and depth will influence the size of the soakage treatment area. Some soil types are not suitable as drainage fields – clay, for example, can cause wastewater to pool on the surface.

Legal requirements

To install an on-site sewage system, you'll need a building consent.

You may also need a resource consent or at least need to meet regional council requirements regarding the discharge of effluent. Some councils will require additional treatment of wastewater with ozone, ultraviolet, filtration or chlorine to make the soakage treatment area safe.

An environmental engineer's report and a soil analysis may be required, to determine where tanks should be sited and where treated waste may be released to, as well as how large the soakage area has to be.

The system will have to be installed by a qualified installer. Look up 'water treatment' using an online search tool. If you have access to a mains sewage system, you can't have an on-site sewage system.


On-site sewage systems can cost many thousands of dollars. As well as the system, you may have to pay:

  • design fees
  • an engineer's fee
  • building and resource consent charges
  • an annual inspection fee (this may be included in the system cost).
  • ongoing maintenance and clean-out costs (this may be a requirement of your building consent).

If you skimp on system size or type, your system may not work effectively. This could endanger health and harm the environment.

TIP - Maintenance requirements

Maintenance can be expensive – factor in the maintenance requirements of the system type and your council when you are making a decision.

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: