When was your septic tank last checked?
Environment Canterbury media release: 13th May 2026
Don’t put your drinking water and family’s health at risk by failing to maintain your septic tank or onsite wastewater system.
It’s estimated there are about 32,900 onsite wastewater systems, or OWMS, often called septic tanks, across Canterbury. That means OWMS service the wastewater disposal needs of around 14 per cent of Canterbury’s population.
Learn more about onsite wastewater management systems.
The most common OWMS for single households in Canterbury is a primary treatment tank (septic tank) and land application system (such as a boulder pit or sand trench). However, some sites have newer secondary treatment systems.
All wastewater management systems need proper installation, operation and maintenance, and if not looked after properly there could be serious consequences. Faulty systems can contaminate the land or drinking water on your property, or even the drinking water supply for your whole community.
Could your septic tank be making you sick?
When it comes to health risks from onsite wastewater, prevention is key.
Problems often begin when people don’t maintain their systems and the tank or gully trap overflows, causing ponding of wastewater on the land surface. Risks are greater where land has moved due to landscaping, vehicle movements or the installation of children’s play equipment.
PHF Science Senior Groundwater Scientist Bronwyn Humphries says there is anecdotal evidence linking seepage with children’s play areas.
“Kids might just assume it’s a muddy puddle, and not bother to tell their parents.
“There was a case in Canterbury where children had sustained gastro for two years before the family finally figured out the kids were playing in a waste disposal field near their playground. It can take people a long time to realise something is wrong.”
Onsite wastewater can also enter groundwater, from which much of Canterbury’s drinking water is sourced. Weather can exacerbate the issue; prolonged dry periods followed by heavy rainfall events can cause wastewater to rapidly infiltrate groundwater, significantly increasing the risk of contamination.
Septic tank safety - tips from Heath New Zealand/Te Whatu Ora
Keep children away from onsite wastewater systems and remind them to wash their hands in warm water with soap after playing outside.
Grow fruit and veggies away from onsite wastewater, including the plumbing and land application area.
Only healthy adults should carry out work on septic systems. Protect yourself from getting sick when doing maintenance work by:
- wearing appropriate protective equipment such as boots, gloves, and goggles to minimise your contact with sewage
- covering cuts or grazes with waterproof dressings
- minimising splashes
- following the manufacturer’s instructions
- washing work clothes separately in hot water immediately after completing work
- washing hands, forearms, and any other parts of your body that could have been in contact with sewage with soap and warm water
- using a bleach solution or other disinfectant to wipe down any surfaces such as door handles or flashlights that might have been contaminated with sewage.
How often should septic tanks be serviced?
Most OWMS separate wastewater into liquid that can be dispersed through a land application system and sludge that remains in a chamber. This sludge builds up over time and needs to be periodically removed – typically at least once every three years.
Often, people moving from homes serviced by a council-managed wastewater network to those with OWMS aren't aware of where their wastewater system is, how it works, and the need for servicing.
Real estate agents and lawyers are not legally required to disclose the type of system and the service history, so the impetus is on the property owner to find out all they can – including whether the system requires resource consent.
Signs your system may be failing
Signs of potential problems could include water presenting at the surface and offensive odours – often a strong ammonia or sewage-type smell.
Throwing bleach or antimicrobial cleaning products down your toilet is a big no-no, as it kills off the ‘good’ microbes that help to break down the sludge. You should only flush the three P’s: Pee, Poo and Paper. Items like wet wipes and sanitary pads are designed not to break down and can cause blockages.
PHF Scientist Bronwyn Humphries recommends carrying out a stress test to check whether your tank and land discharge are coping.
This involves running an inside tap for an hour or two, to simulate the household's daily water use.
“Ideally, you’d want to do it in summer, during dry conditions. This makes any surface ponding easy to spot.”
Why septic tank maintenance matters
Regardless of whether your onsite wastewater system is new or old, keeping it well maintained will help ensure any problems are caught early.
As Bronwyn points out, it all comes down to the property owner’s care and knowledge.
“You don’t want your family to get sick, nor do you want to be the cause of a community contamination event, such as if wastewater from your property contaminates a downgradient drinking water well. You also don’t want your system to fail to the point where a complete renewal is required, which may cost between $30,000-$40,000.
“Enteric (occurring in the intestines) illnesses stemming from drinking water supplies that are contaminated by wastewater can include norovirus, which may leave you unwell for several weeks. Longer illnesses are also possible, due to pathogenic organisms like cryptosporidium and giardia.
“The cost to people’s health and your bank account could be very significant if things go wrong. I’d encourage anyone who’s been putting off their tank maintenance to act now.”
Septic tank maintenance tips
- Newer, secondary treatment systems require regular inspection and maintenance at least every 12 months, including sludge removal every three years.
- Older, primary treatment systems (septic tanks) – while not requiring regular servicing – still need to be inspected and have sludge pumped out around once every three years.
- Support the microbial community living in your onsite wastewater system by using low-phosphorous soaps and detergents.
- Be sure not to flush solids such as wet wipes, or aggressive chemicals such as caustic soda.
Water NZ has more information on what you can and cannot flush.
- Space out water use to avoid overloading the system – for example not running your washing machine and dishwasher at the same time.
- If you notice an issue – like a sewage overflow, murky or smelly puddles around your treatment system, or even a broken tank lid – you should get your system inspected immediately by a suitably qualified technician.
Learn more about maintaining your onsite wastewater system, spotting warning signs, and understanding your responsibilities as a property owner.
Before buying a property with a septic tank
Before purchasing property with an onsite wastewater management system, check with a lawyer or estate agent whether the system is consented, and how it's been maintained.
Learn what questions to ask before buying or selling a property with an onsite wastewater system.