Selwyn Water has made every reasonable effort to provide complete and accurate information in the Nitrate Level map. However, we take no responsibility or liability for any errors and omissions, or for the use or misuse of any of the information in the Nitrate Level map.
Water quality test results
The Water Services Authority, Taumata Arowai, is responsible for setting the New Zealand Drinking Water Standards, which specify the maximum amount of different chemical determinands (specific substances and contaminants) that are safe to have in our drinking water. These are known as Maximum Allowable Values (MAV).
Selwyn Water monitors for chemical profiles in drinking water. The Annual Chemical Analysis Report with these test results are provided in the following link.
2025 Annual Chemical Analysis of Water Sources
Nitrates
Nitrate is a chemical made of nitrogen and oxygen. It can occur naturally and is found in low concentrations in water and soil, but it can also occur in higher concentrations in some water sources. There are a number of factors that can influence nitrogen concentrations, these can include land use and environmental factors.
This page shows Selwyn Water's drinking water supply testing results for nitrates, displayed as NO3-N (nitrate-nitrogen) measured in g/m3. New Zealand’s water legislation sets required units as “mg/L”, which is a different way of expressing g/m3.
Select a scheme boundary on the map below to see the latest nitrate‑N result for each water source. Results are updated monthly.
When you click on the map the colours shown represent the nitrate level compared to the Maximum Acceptable Value (MAV) of 11.3 g/m3. The colours show nitrate levels of:
| value less than 5.65g/m3 |
values between 5.65g/m3 & 11.3g/m3 |
value more than 11.3g/m3 |
Explore nitrate-N readings over time
The Water Scheme filter applies to the table and graph below.
Source ID Code is the public registration code on the public register (Hinekōrako) held by Taumata Arowai.
Nitrate testing and frequency
Taumata Arowai, the drinking water regulator, is responsible for setting the New Zealand Drinking Water Standards, which specify the maximum amount of different chemical determinands, including nitrates, that are safe to have in our drinking water. This is known as the Maximum Acceptable Value (MAV).
Nitrates are tested under the following expressions:
| Nitrate expression | Nitrate expression description | The Maximum Acceptable Value (MAV) |
|---|---|---|
| Nitrate or NO3 | Both the nitrogen and oxygen atoms that make up a nitrate molecule | 50g/m3 |
| Nitrate-nitrogen or NO3-N | Just the nitrogen atoms that make up a nitrate molecule | 11.3g/m3 |
All nitrate samples are analysed in an IANZ Accredited Laboratory. The detection threshold for nitrate nitrogen is 0.005 g/m3.
Our frequency for testing nitrate and nitrate-N is determined with accordance to the Drinking Water Quality Assurance Rules set out by Taumata Arowai. Large drinking water supplies, serving more than 500 people are tested monthly. Drinking water supplies, serving less than 500 people are tested annually. Any drinking water supply, regardless of size is tested monthly if results exceed 50% of the MAV.
For further information refer to the Taumata Arowai website.
You can also read the latest information from Health New Zealand on nitrates in drinking water.
Nitrates in Selwyn waterways
Environment Canterbury is responsible for ground water and waterway in Canterbury and has more information about its long-term plans for managing and reducing nitrate levels in Canterbury waterways. Building on the Council’s commitment to investigate low‑nitrate water sources, Selwyn Water will continue this commitment by investigating the feasibility of centralising some plains water sources to a low‑nitrate supply.