Some substances if they make their way into our drinking water can be harmful to our health. They could increase the risk of several types of cancers for those who are exposed over a long period of time.

Arsenic, asbestos and nitrates are substances that can make their way into our drinking water from the surrounding environment and can reach levels which are considered dangerous to health.  


Long term exposure to arsenic in drinking water can cause some types of cancers. Drinking water contaminated with nitrate and asbestos are not confirmed to cause cancers, but caution should be taken whenever water is contaminated with potentially harmful substances. 

How do these substances get into our drinking water?

Arsenic is naturally found in low levels in rocks and soil. It is used as a timber preservative and fertiliser. It can seep into soil and make its way in to surrounding waterways and drinking water. Elevated concentrations of arsenic is found in our soil due to past use of herbicides and pesticides, such as sheep dip, and in our waterways in geothermal and mining areas.

Asbestos is a naturally occurring mineral which comes from rocks and may enter our water by the process of erosion. The pipes that carry our drinking water were made of a concrete that contains asbestos until the mid- 1980s. No new pipes with asbestos have been installed in Aotearoa New Zealand since 1986, but many of the old pipes are still in use. When asbestos pipes get older some of the asbestos fibres can shred from the concrete into the drinking water. Asbestos can also get into the water from landfills and construction sites.

Nitrates are found naturally in the environment and are essential to all living things, but too nitrate much in the water can be harmful to health. Nitrate dissolves in water and can easily travel through soil to enter the groundwater. Water contamination often comes from runoff from farming, including fertilisers and animal waste, but can also enter from wastewater and septic tanks.

What is the potential harm from these substances?

Most New Zealanders are not exposed to levels of these substances that cause any health problems.  But if people swallow high levels every day for many years, they may cause serious health problems. 


Exposure to arsenic can cause cancers of the lung, bladder and skin. In Aotearoa New Zealand the maximum safe limit for arsenic in drinking water is 0.01mg/litre, anything above this level is considered harmful. Levels of arsenic in the drinking water are monitored by our new national water regulator Taumata Arowai.

Asbestos is known to cause cancers of the mesothelium, lung, larynx and ovary, when it is inhaled (breathed in). Less information is known about what the danger is of ingesting (drinking) asbestos in the drinking water. Currently there is no consistent evidence  that drinking asbestos increases the risk of developing cancer. But, because drinking asbestos daily for long periods of time may be bad for our health, asbestos cement water pipes are gradually being replaced. 


Nitrates are not proven to be carcinogenic. However, when nitrates are metabolised, they can react with other substances to form compounds widely shown to be carcinogenic. Some studies have found that drinking water with elevated levels of nitrates for years may increase the risk of bowel and other cancers, while other studies report that it does not.  As the link is uncertain, a cautious approach should be taken when there is a possibility a substance may increase cancer risk. Aotearoa New Zealand has a maximum level of 11.3mg/L of nitrates in the drinking water. There is on-going scientific debate about whether this level is safe. 


Council and other public water supplies are required to be tested, treated, and comply with new Drinking Water Standards . Taumata Arowai, is the new water regulator which is responsible for our water suppliers. They are working to ensure all communities have access to safe, reliable drinking water.  


If your drinking water comes from a private source such as a bore or well on your property make sure it is constructed in a safe spot away from septic tanks, and areas accessed by animals and is protected and maintained regularly. You should get the water tested by an accredited laboratory regularly (e.g. annually) to make sure it does not contain harmful levels of any of these substances. People using bore water in the Waikato region should have their water tested more often for arsenic. Further information about household water supply can be found here.

Boiling water does not effectively remove any of these substances, and general home water filters cannot effectively remove arsenic or nitrates. You can purchase and install home nitrate filters, which are expensive, but have been shown to be effective when used correctly. If your water supply has high levels of any of these substances, use alternative sources of drinking water and seek further advice. Do not drink bore water that is contaminated.

For further information of drinking water and potential carcinogens:


Review of nitrates by the Office of the Prime Minister's Chief Science Advisor

This content has been peer reviewed by Dr Tim Chambers,  BPhEd(Hons 1st Class) BA PhD (Public Health), Senior Research Fellow; HEIRU

Last updated: December 22, 2022