How to make sure your drinking-water supply is safe

How to make sure your drinking-water supply is safe, responding to a positive E-coli result and other water contamination risks

In this section:

Water used for drinking, teeth cleaning, hand  washing, bathing, showering, food preparation, and cooking needs to be free from harmful germs and chemicals. Water contamination can cause illness (like diarrhoea and vomiting) that can be particularly dangerous for infants, very old people or people with low immunity.


How to make sure your supply is safe

A safe water supply has the following components:

1. Stopping contaminants getting into your water in the first place.

2. Removing dirt and killing any germs that do get into your source water (such as installing some form of water treatment device).

3. Stopping water from getting re-contaminated in your tank or pipework.

4. Checks, monitoring and maintenance to make sure the above points are happening.

Water supplies can very easily become contaminated and unsafe and there are a wide variety of causes, including change in land use affecting source water quality, treatment failure or ineffective treatment, and the entry of raw (untreated) water into the tanks or pipework.

The most common contaminant of drinking water is animal or human faecal matter.  Generally, water that has come from a deep bore is safer than that from a river or stream, or water from a roof or spring, which can be contaminated by dirt and faecal matter running off the surrounding environment.

Rainwater quality can be improved if the system is well maintained and steps are taken to protect the supply from contamination during collection and storage, such as installing a first flush diverter.

Underground supplies (wells, bores and springs) vary in quality although protection at the source will often reduce the likelihood of contamination. The degree of contamination in rivers and streams can vary but will undoubtedly increase after periods of rainfall.

Treatment systems are generally required for all types of supply sources with the exception of very deep groundwater sources that have been demonstrated to be secure through extensive testing and monitored closely. The most appropriate treatment system for a small supply is a filter and ultraviolet system supplied and installed by a reputable equipment supplier.


Responding to a positive E. coli result

The presence of E. coli (Escherichia coli) in drinking-water (a transgression) is a sign that it has been contaminated with faecal matter. Therefore, there is the potential for the water to contain pathogenic micro-organisms, which is a risk for anyone drinking the water. If you are responsible for a water supply and receive a positive E. coli result from a drinking-water supply the following steps should be followed:

  1. Immediately consider whether to warn consumers in the affected area not to draw water until further notice, and issue a boil water notice – See Drinking-water in an emergency.
  2. Immediately consider whether to cease abstraction and switch to an alternative source of drinking-water until water of acceptable quality can again be supplied.
  3. Identify what is causing the contamination, and how to stop the contamination – See Inspection checklist.
  4. If the contamination can be stopped, drain and flush the affected part of the distribution system, considering the need to flush with elevated chlorine concentration - See Disinfecting water storage tanks.
  5. Organise follow up samples to check that the supply is safe to use – See Monitoring the quality of your water supply.
  6. Notify consumers that they will need to flush their taps and storage tanks until good quality water can again be drawn.
  7. If the contamination is a more permanent problem, investigate and develop an alternative supply and/or treatment.


Drinking-water in an emergency

A range of hazards and emergencies may contaminate your water supply and make it unsafe to drink or use.

The Ministry of Health provides broad advice on drinking-water in an emergency and will provide updates in a water contamination event to provide specific details relating to the incident visit the Water contamination events website here.


Arsenic in ground and surface water supplies

Arsenic is a naturally occurring chemical element, present at low levels in all soil, water, plants, animals, and foods. The main way people are exposed to arsenic is through the small amounts of this element that are naturally present in food and drinking water. 

Elevated arsenic levels in ground and surface water can occur throughout New Zealand, in geothermal areas of the North Island and mining areas. The level of arsenic may be different in each ground or surface water source and may also change over time. 

The Drinking-water Standards for New Zealand sets a maximum acceptable value for arsenic at 0.01 mg/L. Long-term exposure, such as swallowing moderate amounts of arsenic every day for many years is associated with some types of cancers and other serious health conditions. 

When a new water source in geothermal areas e.g. Rotorua and Taupō is being considered, the level of arsenic should be checked before resources are invested in developing the supply for drinking and sanitation purposes. 

If you have tested your water supply and found arsenic at levels above 0.01 mg/L we recommend using an alternative water source or treating the water used to drink, clean teeth, hand  wash, bath or shower, and to prepare food.

Treatment can be costly, but arsenic can be removed from a water supply. There are filters available however, we suggest speaking to a reputable equipment supplier in the first instance because water chemistry can affect filter performance and may not be suitable in all situations. 


Algal blooms in surface water supplies

Algal blooms (Cyanobacteria) can produce cyanotoxins and these can have serious health effects. Activated carbon and reverse osmosis filter systems are able to remove cyanotoxins from water, but it would be preferable to avoid abstracting (taking) water during cyanobacterial blooms or switch to an alternative source. Boiling the water does not remove the toxin.

For information on algal blooms and how they are monitored in the Bay of Plenty and Lakes District visit the Bay of Plenty Regional Council website here

Current health warnings are listed on our Health warnings page here.


Sampling and analysis

All supplies should undertake some water sampling (monitoring) to ensure that the barriers and treatment in place is working and no contamination of the supply is occurring prior to reaching the consumer.

The level of assurance gained from monitoring is related to the number of samples that are taken over a monitoring period, so low sample numbers gives a low level of assurance.

The main consideration for monitoring is for microbiological safety because illness is more likely to be dramatic and widespread when due to microbiological contamination. Nonetheless the supplier should also know whether there are chemicals of concern in the supply, but generally this may be determined by one-off sampling. Check that the laboratory you use is recognised by the Ministry of Health for the analysis they are going to do.


Drinking-water advice and information

Our Health Protection Officers can provide information about possible sources of contamination and the risks. They can also provide advice on how to make drinking-water safe in the short and long term, when sampling and testing may be useful and what the results tell you about the safety of the water supply. If you are concerned that your water supply may be unsafe or may be causing you to be unwell contact us.


For more information: