Drinking Water Quality Testing - Hardness

This page shows Selwyn District Council's drinking water supply testing results for total hardness displayed as CaCO3 measured in g/m3.

The content of the mineral's calcium and magnesium in water are measured to determine the hardness of drinking water. Higher levels of calcium and magnesium will make water harder. More than 90 percent of water supply zones in New Zealand receive water that, according to a commonly used hardness scale is classified as soft.

Certain household appliance installation instructions ask if your water is hard or soft. This is because hard water may cause scale deposition. It will also result in high soap consumption and subsequent "scum" formation. On heating, hard waters form deposits of calcium carbonate scale.

For hardness, the aesthetic value for drinking water is considered to be ≤200 mg/L hardness (total) (Ca + Mg) as CaCO3.

Select a scheme boundary to find the most recent total hardness result for each source. Data is updated monthly.

When you click on the map the colours shown represent the total hardness compared to the aesthetic value for drinking water ≤200 mg/L hardness (total) (Ca + Mg) as CaCO3 and against a hardness scale. This shows Selwyn drinking water sources are defined as soft or at the lower end of the moderate hard scale.

Hardness description Hardness level
Soft water 0-60 mg/l as CaCO3
Moderate hard 61-120 mg/l as CaCO3
Hard 121-180 mg/l as CaCO3
Very hard more than 180 mg/l as CaCO3
180 mg/l is the New Zealand Aesthetic limit
Hard water more than 200 mg/l as CaCO3

Explore hardness readings over time

The Water Scheme filter applies to the table and graph below.

Explore hardness readings over time

The Water Scheme filter applies to the table and graph below.

Total hardness testing and frequency

The drinking water authority Taumata Arowai have published aesthetic values for New Zealand drinking water in the Aesthetic Values for Drinking Water Notice 2022.

All samples are analysed in an IANZ Accredited Laboratory. The detection threshold for total hardness is 1.0 g/m3 as CaCO3. New Zealand's water legislation sets required units as "mg/L", which is a different way of expressing g/m3.

There is no mandatory frequency for testing the total hardness of drinking water. Selwyn tests annually.