Water chlorination

Christchurch has high-quality drinking water drawn from deep aquifers. Chlorination provides an extra level of protection against contamination.

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 Where we have chlorinated

About our water supply

Tap water in Christchurch city is clean and safe to drink. The quality of our groundwater remains excellent, and we test it daily to ensure it is free of bacterial contaminants.

A low level of chlorine is used to provide an extra layer of protection against potential contamination within our network of pipes. 

If we detect issues anywhere in the network, we may temporarily increase chlorine levels as an extra safety barrier to protect public health.

The future of chlorine in Christchurch

New Zealand’s drinking water laws make it mandatory for all public water supply networks to be treated with chlorine unless an exemption is obtained from the national water regulator - Taumata Arowai.

In November 2023 Taumata Arowai declined Christchurch City Council’s first chlorine exemption applications. This decision signalled the start of a shift in the way we manage water supplies across Christchurch and Banks Peninsula.

Without an exemption in place, chlorine has been introduced to all water supply zones across the city. This is a necessary step to comply with the Water Services Act and the associated rules and standards for community water supplies.

Taumata Arowai's decision helped us understand what would need to occur to be successful in any future applications. Given the complexities of the supply and the amount of capital investment required for Christchurch City, it will be a challenge to ever be successful in a residual disinfection exemption.

The Council is prioritising the installation of continuous monitoring equipment and the replacement of the existing temporary chlorination system.

Why we chlorinate

New Zealand’s drinking water laws make it mandatory for all public water supply networks to be treated with chlorine unless an exemption is obtained from the national water regulator - Taumata Arowai.

The Drinking Water Standards(external link) have been issued by Taumata Arowai under the Water Services Act. They are accompanied by Drinking Water Quality Assurance Rules.(external link) 

The standards specify:

  • Maximum amounts of substances, organisms, contaminants and residues that may be present in drinking water.

The rules specify:

  • Requirements that drinking water suppliers must meet as part of their responsibility to demonstrate that they are producing safe water.
  • Water quality operational requirements.
  • Criteria for demonstrating compliance with standards and demonstrating that MAVs for determinants in the Drinking Water Standards are not being exceeded.

The impact of Havelock North's drinking water contamination

In 2016, the water supply in Havelock North was contaminated with campylobacter. Four people died and around 5000 people fell ill.

Following this incident, the Government carried out an inquiry and established the Three Waters Review to look at how to improve the regulation and delivery of drinking water, wastewater and stormwater – the three waters.

We responded to the review by evaluating the condition of our below-ground well heads (the source of contamination in Havelock North) and began an extensive improvement programme.

An independent assessment found the well heads did not meet standards and the Drinking Water Assessor advised the Christchurch water supply was no longer provisionally secure.

The resulting loss of the secure status of our water supply forced us to make the decision in early 2018 to temporarily chlorinate our water supplies.

If we had not made that decision, the Medical Officer of Health would have required us to chlorinate. Temporary chlorination of much of the city’s water supply began in March 2018.

The Government continued work to review and make changes to three waters across New Zealand and in late 2021 new legislation – the Water Services Act 2021 - was passed. This established Taumata Arowai - the new water regulator.

How we chlorinate

The Christchurch water supply is made up of several zones that operate independently of each other. 

The chlorine dosing takes place at pump stations across the city. From the pump station, the water flows directly out into our water reticulation system across the city.

There are some pump stations where we still need to use unsecure wells while we drill new deeper ones, and others where we use suction tanks that have deteriorated over time. These pump stations are being dosed at 0.5 or 1 ppm, depending on whether the first customer is within one or two minutes of the pump station.

Water treatment in Christchurch.

Drinking water in Christchurch is being treated with chlorine.

How it works

Water chlorination is the process of adding chlorine to the water supply to keep it safe from harmful bacteria. Sodium hypochlorite is used in Christchurch. 

It can be added as a precaution after routine work, such as reservoir cleaning, or as a result of finding bacteria in the water supply during routine water sampling.

We previously put chlorine in the water immediately after the Canterbury earthquakes and intermittently when bacterial transgressions are detected. 

Chlorine is a powerful oxidising agent. As it travels through the system, it will react with any organic matter, such as slime build-up in the pipes.

It might also react with iron in the old cast-iron mains and it will react with any other organic material it comes across.

Upgrading our network

Since 2018 we have upgraded most of our well heads, installed ultra-violet disinfection at six wells, decommissioned a number of shallow wells and drilled ten new ones, but there is still further work being done.

One of the biggest ongoing risks to our water supply relates to the condition of some of the reservoirs and suction tanks used to store drinking water. There are risks such as cracks in the roofs or in below-ground walls of the reservoirs which could allow contaminated water to get in.

To address this, we have a reservoir and suction tank inspection and repair programme underway, which involves carrying out detailed inspections of our reservoirs and suction tanks to check what will be required for them to be considered ‘demonstrably safe’.

Inspecting and repairing reservoirs can only take place during the winter months when a small number of reservoirs can be taken out of service as they aren’t needed to supply water. The inspections inform the scope of repair work so it is difficult to know exactly when the work will be complete, but we estimate the programme will take about five years to complete.

The other unacceptable risk to our water supply network related to backflow occurring where commercial and industrial properties have inadequate, faulty or no backflow prevention devices installed.

Backflow occurs when the water flows backwards from a property’s plumbing and into the public water supply network. It can happen, for example, if the pressure drops in the network and causes water – and potentially chemicals and other contaminants – to be sucked back into the public supply.

A considerable work programme to install backflow prevention devices at customer connections has addressed high-risk connections. The majority of medium-risk connections have also been addressed, and we are confident that backflow is no longer an unacceptable risk to the safety and security of our water supply.

Find out more. 

Reservoirs and suction tanks

There’s a risk of contamination in some of our reservoirs and suction tanks due to animals potentially getting into tanks through unprotected vents, overflows or open hatches, and due to cracks in some structures potentially allowing contaminated water to enter.

The work we’re undertaking is to install hatch alarms where these are absent, repair poor-condition roofs, hatches and seals, and make sure we have mesh in place over vents and overflows.

We carry out regular assessments of all of our reservoirs and suction tanks and undertake any upgrades needed to bring them up to standard.

Health and wellbeing

Chlorine kills the bacteria that can get into water supplies and spread disease, helping ensure supplies are safe to drink.

Chlorine has been used safely all around the world for about 120 years. It keeps millions of people all over the globe – including in most of New Zealand – safe from waterborne illness.

Tap water is still safe for people and pets to drink. The quality of the groundwater remains excellent and we test it daily to ensure it is free of bacterial contaminants.

Drinking water

There are no known health impacts from drinking water treated with chlorine and it is safe for pregnant women to drink. The quality of the groundwater remains excellent and it is tested daily to ensure it is free of bacterial contaminants.

The amount of chlorine dosed into the water supply will be carefully managed to ensure levels of chlorine in the water people drink are minimised. 

Taste and smell

There are 53 pump stations across the city. While you will be primarily supplied by one pump station, if you are midway between pump stations, sometimes you may be getting the water from one pump station and sometimes the other. The level of chlorine in those pump stations may be different. If you live close to the pump station, the chlorine dose you get coming out of your kitchen tap will be higher than if you live further away.

If you are concerned about the taste, you can keep drinking water in an open jug in the fridge. The chlorine taste will dissipate naturally over a few hours.

Chlorine and any associated by-products can be removed by using a granulated, activated carbon (GAC) filter. These are available from hardware supply stores and water filter companies.

Skin conditions or sensitivity to chlorine

Chlorine can be an irritant for existing skin conditions such as asthma or eczema. If you feel your skin getting dry or itchy, use moisturiser after having a shower or bath, or if you notice increased skin irritation, asthma symptoms or other symptoms, seek medical advice from your GP. 

To minimise exposure to chlorine, try bathing at times of low water demand – in the middle of the day on weekdays, early in the morning (before 7.30am), or late in the evening (after 9.30pm).

In Canterbury, you can call your usual GP’s number after hours and your call will be put through to a nurse who can provide free health advice. You can also contact Healthline any time for free health advice on 0800 611 116.


There are no known health impacts from drinking water effectively treated with chlorine. The use of filters will mitigate any risks for those on dialysis. This is arranged by Te Whatu Ora - Waitaha Canterbury. 


The International Agency for Cancer Research (IARC) does not believe chlorinated water is either a probable or even possible, cause of cancer.

The Journal of the National Cancer Institute acknowledges that “water chlorination is one of the major disease prevention achievements of the 20th century”, and that it “has become the principal means of effectively reducing waterborne enteric diseases”, which the World Health Organisation has stated account for a significant number of deaths every year, even in developed countries.

Health advice

In Canterbury, you can call you usual GP’s number after hours and your call will be put through to a nurse who can provide free health advice. You can also contact Healthline any time for free health advice on 0800 611 116.

Looking after pets and fish

Treated water is safe for household pets such as cats and dogs to drink.

If you have fish in outside ponds you will need to either turn down in-coming water to an absolute trickle (this dilutes the chlorine level to a safe amount for your fish) or fill up drums of water and let them sit for at least 24 hours before using (the UV of the sun evaporates chlorine).

For fish tanks or bowls inside, fill up a container of water, let it sit for at least 24 hours and then only replace 1/3 of this water at a time with what is in the tank already. If you’re still worried, you can buy de-chlorinating kits (sodium thiosulfate) at pet supply stores.

We use chlorine, not chloramine (chloramine is more toxic to fish).

Hot water cylinders

A hot water cylinder.Chlorination has been linked with an increase in hot water cylinder failures. We've answered some of the frequently asked questions about the issue.

Why hot water cylinders fail

Christchurch has historically had issues with hot water cylinder failure. Certain areas of the city, such as Cashmere, have been affected more than others.

There are multiple factors that can lead to a hot water cylinder leaking, and then needing to be replaced. These include the chemical composition of the water, the age of the cylinder, the type of cylinder, whether there is any debris in the cylinder, and the particulars of the installation.

We have been working with manufacturers and it appears the majority of the cylinders that are failing are reported as older, low-pressure copper cylinders.

Our response to the 2018 University of Canterbury report on the issue

We agree that the presence of chlorine in the water, along with the other factors noted in the report (such as temperature, details of installation and water chemistry), have contributed to the observed pitting corrosion.

We similarly agree that pitting corrosion of hot water cylinders is likely to become more frequently observed.

Why other parts of New Zealand are not having large numbers of hot water cylinder failures

The chemical composition of water supplies around the country is quite different.

Places like Auckland and Wellington use surface water (from rivers and streams), which generally develops a protective film when in contact with copper plumbing.

In Christchurch, we use groundwater from aquifers. Pitting corrosion leading to pinhole failure happens more commonly in some bore water (underground) supplies.

In Hastings and Napier, for example, they have bore water supplies and also experienced a rise in hot water cylinder failures following the chlorination of their water networks. Napier and Hastings found the number of failing cylinders decreased over the year after chlorination was introduced.

If you need to repair or replace your cylinder

We understand that having to replace your hot water cylinder is a costly exercise and we also acknowledge that there has been an increase in hot water cylinders failing since the introduction of chlorine.

It’s important to remember there are multiple factors as to why pitting occurs in hot water cylinders. These include the chemical makeup of the water, the age of the cylinder, the type of cylinder used, the debris in the cylinder, and the quality and thickness of the copper used. Because of this, the Council is not compensating property owners.

We are continuing to monitor the situation and work with manufacturers and suppliers to gain further understanding. Most of the city is currently being dosed at 0.2 parts per million chlorine, which is a fifth of the dose we first started treating the drinking water with.

There are specific cylinders that are sold for areas where pitting corrosion is prevalent. If you are replacing your cylinder, talk to your plumber about the best option for your area. Some options include enamel-lined steel or stainless-steel cylinders.

Affects on gas systems or pipes

There is no evidence of an increased number of failures in copper heat exchangers in gas continuous-flow units, copper water pipes, or other fittings.

More about hot water cylinders and chlorine.(external link)

For more information

If you have any unanswered questions about water chlorination, you can email them to watersupply@ccc.govt.nz or call 03 941 8999 or 0800 800 169.