Maintaining your on-site sewage system

Last updated: 9 August 2023

onsite sewage systems

A well-maintained on-site sewage system will work well and be safe for you and the environment.

Maintain your sewage system

On-site sewage systems such as septic tanks need regular maintenance – some you can do yourself, and some you'll need professional help with.

Sustainable wastewater management: A handbook for smaller communities on the Ministry for the Environment website has useful information.

Why maintain your on-site sewage system?

Keeping people healthy

Maintaining your on-site sewage system is important for the sake of your health and the health of others in your neighbourhood. A poorly maintained system can contaminate bore water, groundwater, and waterways, potentially spreading infection and disease.

Household wastewater may contain:

  • viruses which can cause illness such as viral gastroenteritis or hepatitis A
  • bacteria such as campylobacter and salmonella
  • protozoa such as giardia and cryptosporidium
  • worms such as hookworms and roundworms which can cause fever, aches and chills.

Wastewater may also contain pollutants such as nitrates and phosphates that can cause toxic algal bloom in waterways.

Meeting legal obligations

You have legal obligations if you own a property with an on-site sewage treatment system.

You cannot legally sell or rent your property if the on-site sewage system is not operating correctly.

If you contaminate your own, or neighbouring properties and waterways, you can be prosecuted.

If you develop your property by adding another dwelling or adding to the existing dwelling, you must comply with the rules covering sewage treatment. If your soakage treatment area is built over or disturbed, and you do not rectify it, you may be liable to prosecution.

If you are a tenant, it is the landlord's responsibility to keep the system maintained in safe working order, as part of the general maintenance of the property.

Don't overload your system

The best way to keep maintenance to a minimum is to not overload your on-site sewage system with too much wastewater.

Most of your wastewater comes from your washing machine, toilet and bathroom. It's possible to reduce this load on your sewage system by:

Upgrade your system if necessary

If you've reduced your water use and your on-site sewage system still isn't coping, you'll need to consider an upgrade.

Multi-chamber on-site sewage treatment systems can be added to existing single septic tanks to increase capacity and speed up the sewage treatment process. Or your soakage treatment area may not be big enough or sufficiently well-drained to cope.

Seek expert advice to determine where the problem is. Look up water treatment, or septic tanks services using an online search tool.

Be careful what you put into the system

What you put into your on-site sewage system will determine how much maintenance it needs. To keep it working effectively:

  • limit the number of loads of laundry in one day to no more than one or two
  • don't empty large quantities of water into the system all at once, such as from swimming and spa pools
  • keep rainwater out of the system
  • don't put chlorine bleaches or other strong chemicals, engine oils, coffee grinds, tea bags, tissue of more than two ply, disposable nappies, tampons, paints, thinners, dental floss, adhesive plasters and bandages or foodstuffs into the system – these things will make the system less efficient and can stop it from working at all, which means you'll have unsafe effluent discharged into your soakage treatment area
  • don't put anything toxic into it because the toxins will impair the effluent treatment process and may end up in your system, unable to be broken down.

TIP - In-sink waste disposal units

In-sink waste disposal units should not be installed to a standard septic tank system as it overloads the treatment stage and increases the frequency of servicing required.

Don't go into the tank

Your on-site sewage system contains toxic gases that could kill you. Never enter a septic tank yourself, and never leave the access hole open.

Regular maintenance

Maintenance contracts

Some sewage treatment systems come with maintenance contracts. These can be expensive – the cost of maintenance is an important factor in deciding what sort of system to use.

Taking up a maintenance contract means your on-site sewage system will be properly maintained without you having to worry about it.

You may be able to check sludge levels yourself.


Tanks should be inspected every six months or so.

If you have a maintenance contract, your contractor will do this for you. Otherwise, you'll need an expert to do it. It is also a good idea to take a soil analysis from your soakage treatment area from time to time to test for contaminants. Look under water treatment or septic tank services using an online search tool.


Tanks will need pumping every three to five years to remove sludge and sediment. The frequency depends on:

  • what goes into the system
  • how many people use it
  • the capacity of the tanks
  • whether you recycle greywater
  • council requirements (some councils specify every two years).

Most councils require you to do a five-yearly pump-out. And, for houses with older septic tanks, the pump-out period is every one to two years.

As with the inspections if you have a maintenance contract, your contractor will do this for you. Otherwise, you'll need an expert to do it. Look up water treatment, or septic tank services using an online search tool.

Protect the soakage treatment area

Protect the soakage treatment area so that the effluent leaving the tank has optimum conditions to break down.

Don't let people or vehicles on it, or anything else that might disturb or compact the surface.

Trees cause problems when planted in soakage treatment areas as their roots can cause pipes to clog: grass and moisture loving plants are good choices instead.

Signs of trouble

Call for expert help if:

  • you notice a disagreeable smell, either from your drains or from the soakage treatment area
  • your toilets flush away slowly or your sinks and baths take a long time to empty
  • your soakage treatment area is permanently wet – particularly where liquid seeps over the surface or if a green algae-like growth is forming (or you have excessive grass growth in dry weather).

If there's a bad smell around your soakage area, it may be clogged with organic material. In this case, it may have to be moved.

If the smell is in your drains or toilet then sewage is backing up.

Some of these problems might indicate that your system doesn't have enough capacity to deal with the wastewater you're putting into it. If that's the case, you'll either need to put less waste into the system or increase its capacity and efficiency.

If you have a maintenance contract, call your contractor about any of these problems. Otherwise, look up water treatment or septic tank services using an online search tool.

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: