LAWA Groundwater Quality - National Picture 2025

Published: 28 September 2025

Why groundwater matters

Groundwater is a vital resource for New Zealand. It is widely used as a source of drinking water, whether by individual households with their own wells, or by communities and cities including Christchurch and Wellington. Groundwater is also used for irrigation and industry, and it feeds many streams and lakes. Understanding the quality of groundwater helps us manage land uses that affect it and the users that depend on it.

Pathogens, nitrate nitrogen (nitrate), and other contaminants can affect the suitability of groundwater for various uses. Groundwater contaminants, especially nitrate and phosphorus, can also affect water quality in spring-fed streams and lakes. Overuse of groundwater in coastal areas can draw seawater into aquifers, rendering the groundwater unsuitable for drinking or irrigation.

What is monitored and shown on LAWA?

LAWA shares groundwater quality data from over 1000 wells across New Zealand. Monitoring is carried out by regional councils and unitary authorities for a range of water quality indicators, including chemical, physical, and bacterial measures. The latest state and trend results use data collected up to the end of December 2024. For the first time, 20-year trends have been analysed at some sites.

What does this national picture cover?

This summary shows the state of groundwater quality and how it has changed over the past 10 years, using the following key indicators: 

• Nitrate nitrogen – an indicator of contamination that can affect human health and the environment.

• Dissolved reactive phosphorus (DRP) – the plant-available form of phosphorus. This mobile form can move through groundwater and reach surface water, where it can impact overall water health.

• Escherichia coli (E. coli) – a reliable indicator of faecal contamination, showing the potential presence of pathogens that pose health risks if ingested.

• Chloride – used to track seawater intrusion and human activities that can affect drinking water quality.

• Electrical conductivity – a measure of total dissolved solids, useful for tracking changes in groundwater quality over time.

Monitored wells tend to be biased towards areas with higher risk or known contamination. Regional councils and unitary authorities often prioritise these wells, rather than using resources to monitor wells in areas of low risk or high quality. This means that the data shown on LAWA is not a complete picture of groundwater quality across New Zealand but it does provide valuable national insights from wells that are regularly sampled.

How is state determined?

LAWA reports the state of groundwater quality using thresholds for each indicator. State is based on median values (or for E. coli, whether it has been detected at all) using five years of data.

For more detail on how state is assessed, see our factsheet.  

Groundwater quality state change over time

The graphs below (Figures 1-5) show that, at a broad national scale, changes in groundwater quality are subtle. The data reveals:

• While most groundwater in New Zealand is of very good quality, some contamination from E. coli and nitrate is present.

• Seawater intrusion is not widespread, however, continued monitoring is important to detect any changes.

Nitrate nitrogen

• Groundwater from around 60% of the wells presented on LAWA has a median nitrate nitrogen concentration greater than 1 mg/L (Figure 1). This generally reflects contamination from human activities. 

• Around 25% of wells are higher than half the drinking-water standard (5.65 mg/L).

• About 7% of monitored wells exceed the drinking-water standard (11.3 mg/L).

• These proportions have remained relatively unchanged over the past 10 years.

• Higher nitrate concentrations are typically found in areas of intensive agriculture, either grazing (for example Canterbury, Southland, Taranaki and Waikato) or vegetable farming (such as Pukekohe in Auckland, and the Horowhenua in Manawatū-Whanganui). A few regions, such as Bay of Plenty, Gisborne and Marlborough, generally show lower concentrations.

State change over time (2015-2024): nitrate nitrogen

Figure 1. Changes in the state of nitrate nitrogen concentrations from 2015-2024 in groundwater from the 756 wells where there were enough data to determine a result each year. The location of these monitoring sites and the current state result (2024) is shown on the map. 

Dissolved reactive phosphorus (DRP)

• DRP can leach into groundwater from farmland, but for the most part, DRP concentrations in groundwater are related to aquifer material. Groundwater in volcanic rocks or limestone aquifers tends to have higher DRP concentrations because the aquifer material contains more phosphorus. This explains many of the higher DRP concentrations in the volcanic areas of Northland, Taranaki and Bay of Plenty, or in limestone aquifers in Hawke’s Bay and northern Canterbury.

• In contrast, alluvial gravels derived from greywacke contain very little phosphorus. Therefore, groundwater beneath the Canterbury Plains and other alluvial gravel plains, such as in Tasman, Marlborough and West Coast, has relatively low DRP concentrations.

• Nationally, DRP concentrations in groundwater have slightly decreased over time (Figure 2).

State change over time (2015-2024): dissolved reactive phosphorus (DRP)

Figure 2. Changes in the state of dissolved reactive phosphorus concentrations from 2015-2024 in groundwater from the 731 wells where there were enough data to determine a result each year. The location of these monitoring sites and the current state result (2024) is shown on the map.

E. coli

• Around half the wells presented on LAWA have had at least one detection of E. coli in the past five years (Figure 3). This shows that groundwater supplies are widely vulnerable to faecal contamination.

• The most common sources of this contamination include grazing animals, farm effluent disposal, and on-site wastewater systems.

• There is little pattern to where E. coli has been detected, except that contamination is most common where the water table is shallow. Even then, detections have been recorded in deeper wells where the water table is more than 50 metres below the ground surface.

Note: Not all wells presented on LAWA are used for drinking water supply, and the results do not represent drinking water quality in New Zealand. The data reflects source water that may be treated before use.

State change over time (2015-2024): E. coli

Figure 3. Changes in the state of E. coli detections from 2015-2024 in groundwater from the 573 wells where there were enough data to determine a result each year. The location of these monitoring sites and current state result (2024) is shown on the map.

Chloride

Chloride levels are compared to thresholds based on the Aesthetic Values for Drinking Water set by Taumata Arowai. Chloride concentrations greater than the Aesthetic Value (250 mg/L) can affect the taste of water.

• Most wells have concentrations well below the taste threshold, and this hasn't changed over time (Figure 4).

• The highest chloride concentrations are in coastal areas. In some cases, a very high concentration may indicate seawater intrusion due to overuse of a well drawing in seawater from offshore.

Other natural factors can also affect chloride concentrations. Aquifer sediments deposited in marine environments may have a high salt content, which can lead to naturally elevated chloride concentrations in groundwater even many kilometres from the coast. Older groundwater and groundwater in volcanic rock aquifers can also have elevated chloride concentrations, though these concentrations are still likely to be well below taste thresholds.

State change over time (2015-2024): chloride

Figure 4. Changes in the state of chloride concentrations from 2015-2024 in groundwater from the 769 wells where there were enough data to determine a result each year. The location of these monitoring sites and the current state result (2024) is shown on the map.

Electrical conductivity

Electrical conductivity is closely related to the total concentration of salts dissolved in the water. The lowest electrical conductivity values reflect groundwater that is strongly influenced by seepage from an adjacent river or where land is not intensively farmed. Values in the two middle categories generally reflect leaching of minerals and nutrients from agricultural land. Groundwater with values in the highest range usually indicate interaction with aquifer material (such as limestone or volcanic rock) or sea water (in coastal areas).

• Over the past decade, there has been a slight increase in the proportion of wells in the two middle range categories, suggesting a possible rise in overall dissolved salt concentrations across the country (Figure 5). The reason for this is not clear. It may reflect ongoing land use intensification over the period, or wells drawing older groundwater due to ongoing pumping.

State change over time (2015-2024): electrical conductivity

Figure 5. Changes in the state of electrical conductivity from 2015-2024 in groundwater from the 790 wells where there were enough data to determine a result each year. The location of these monitoring sites and the current state result (2024) is shown on the map.

Groundwater quality monitoring and reporting

This national picture provides a high-level overview of the state of groundwater quality across five key indicators. The data comes from regional councils and unitary authorities, whose monitoring programmes are designed to assess environmental state and trends at a regional scale.

The role of long-term monitoring

Understanding how groundwater quality is changing over time relies on data from long-term monitoring programmes. LAWA publishes State of the Environment monitoring data from 2004 onwards, covering commonly measured indicators.

There are now approximately 348,000 monitoring records in the LAWA groundwater quality dataset – a reflection of the significant and ongoing efforts and investment by regional councils, unitary authorities, and Earth Sciences New Zealand (formerly GNS).

Explore more on LAWA

The regional pages on LAWA provide more detailed information and allow you to drill down to data from individual wells. Viewing individual well sites allow for more specific insights into state and trends and additional details such as well construction.

Looking at individual site data alongside contextual information at an aquifer and regional level is useful for mana whenua, community groups, landowners, and others interested in how groundwater is tracking. 

To view a specific site:

• On a desktop or tablet: click a site dot on the map on the LAWA topic homepage

• Or navigate by region > aquifer zone > site

For additional context, the organisation responsible for monitoring the well may be able to provide further insights into the data and information shown.