Skip to content

Water quality FAQs

Water quality FAQs

Updated 14 December 2016 3:35pm

About the recreational water quality programme

Weekly surveillance

Overall water quality grades

The good, the bad and the ugly: the causes of poor water quality

About the recreational water quality programme 

How do I find out if it is safe to swim at my favourite swimming spot?

During the official summer swimming season (December to March) Wellington’s regional, city and district councils monitor the quality of water at popular beaches and rivers each week.

The results from this monitoring, including any current water quality warnings, can be found by viewing our interactive water quality map.

Why shouldn't I swim after rain?

Water quality in our rivers and at our beaches is generally pretty good over summer, except in poor weather conditions. Heavy rain flushes contaminants from urban and rural land into waterways and we strongly advise you not to swim for at least two days (48hrs) after heavy or prolonged rainfall – even if a site generally has good water quality.

Remember – Wait two days after rain before you swim again!

Why and what we monitor

We monitor popular beach and river swimming spots to inform people of any potential risks to their health from disease causing organisms (pathogens) in the water. At river sites we also assess the risk from toxic algae which can produce toxins that are harmful to humans and animals. For more information on toxic algae click here.

Water that is contaminated by faecal material can contain many different types of pathogens such as bacteria and viruses which can make people sick. The safety of water for swimming is determined by measuring ‘faecal indicator bacteria’ (enterococci in coastal waters and E. coli in rivers) which indicate likely levels of pathogens in the water. Results are then compared to national guidelines.

While we’re out there we also measure other things such as water temperature, the amount of seaweed or algae cover, and water clarity.

When do we monitor?

We monitor weekly from the start of December until the end of March. If high bacteria counts are recorded sampling continues on a daily basis until bacteria counts return to safe levels. Refer to the Weekly surveillance section for more information.

Where do we monitor?

We currently monitor 65 beach and 25 river sites across the Wellington region.

Our monitoring is focussed on sites where people most like to swim. Such sites are often lowland sites, close to populated areas. Some more remote yet very popular areas – such as the upper reaches of the Otaki and Waiohine rivers – are only monitored monthly because historical monitoring results indicates the health risk is likely to be low.

Who’s involved?

Recreational water quality monitoring in the Wellington Region is a joint effort involving GWRC and local councils, in particular Kapiti Coast District Council, Porirua City Council, Hutt City Council and Wellington City Council. Regional Public Health provides advice on human health risk and health warning requirements.

Weekly surveillance 


National guidelines provide trigger levels that allow us to assess individual monitoring results and determine when management intervention is required. The trigger levels and corresponding management responses are shown in the table below.

Surveillance, alert and action trigger levels for beaches and rivers as specified by national guidelines

Mode Trigger level   Management response
  Beaches – Enterococci (cfu/100mL) Rivers – E. coli (cfu/100mL)  
Surveillance Single sample ≤140
Single sample ≤260 Routine monitoring
Alert Single sample >140
Single sample >260 and ≤550 Increased monitoring, investigation of contaminant source and risk assessment
Action Two consecutive samples within    24 hours >280
Single sample >550 Public warnings, increased monitoring and investigation of contaminant source

When water quality falls in the ‘surveillance’ category this indicates that the risk of illness from swimming is low. If water quality falls into the ‘alert’ category, this indicates an increased risk of illness from swimming, but still within an acceptable range. However, if water quality enters the ‘action’ category, then the water poses an unacceptable health risk from swimming.

What do we do when water quality doesn’t meet the guidelines?

Sites that exceed the ‘alert’ or ‘action’ trigger levels are re-sampled on a daily basis until results return to a safe level. When a site exceeds the ‘action’ trigger level, health warning signs are erected and the public informed that it is unsafe to swim through channels such as council websites and local newspapers. An investigation of contamination sources at the site is also undertaken.

The only time a warning is unlikely to be issued is when an action level result follows heavy rainfall. This is because it is widely accepted that rainfall often leads to elevated bacteria counts in rivers and the coast. For this reason we advise people to avoid swimming and other contact recreation activities for up to 48 hours after heavy or prolonged rainfall. Click here to find out why rainfall is linked with high bacteria counts.

Overall water quality grades 

The technical term for the overall water quality grade is ‘Suitability for Recreation Grade’ (SFRG), and describes the likely health risk from direct contact with the water at any given time. It only relates to health risk from faecal contamination of water from disease causing organisms  (ie, it doesn’t include the potential health risk from toxic algae that may be present) and is made up of two components; an assessment of the catchment area for sources of faecal contamination (ie, farmland runoff, stormwater discharges or large waterfowl populations) and monitoring results from the previous five summers. These grades are determined using the method outlined in national guidelines.

What do the grades mean?

There are five grades, ‘A’, 'B', 'C', 'D' and ‘F’. The risk of becoming sick from contact with the water at a site increases as the grading shifts from ‘A’ to ‘F’.

Overall water quality grade SFRG Health risk Explanation
A Very good Very low risk of illness The site generally has excellent water quality and very few potential sources of faecal pollution. Water is considered suitable for swimming almost all of the time.
B Good Low risk of illness The site is considered suitable for swimming most of the time, although swimming should be avoided during or following heavy rain.
C Fair Moderate risk of illness The site is generally suitable for swimming but because of the presence of significant sources of faecal contamination, contact with the water should be avoided during or following rainfall, or if there are signs of pollution such as discoloured water, odour or debris in the water.
D Poor Caution The site is susceptible to faecal pollution and water quality is not always suitable for swimming. During dry weather conditions ensure that the location is free of signs of pollution such as discoloured water, odour or debris in the water, and avoid swimming at all times during and for two days following rainfall.
F Very poor Unsuitable for swimming The site is very susceptible to faecal pollution and water quality may often be unsuitable for swimming. It is generally recommended to avoid swimming at these sites.

My local swimming spot is graded D (or poor) – does that mean I can't swim there?

Not necessarily. It means that there is an increased risk of getting sick compared to a site that has a better grade. Water quality at sites graded poor will not always exceed trigger levels for safe swimming but will do so more frequently than other sites. As a rule water quality is most likely to be affected during and up to 48 hours after rainfall, but sites graded poor are also more likely to be unsafe for swimming during dry weather. GWRC and local councils are currently investigating the sources of faecal contamination at sites graded D (or poor) so that water quality can be improved.

Are the grades for beach and river sites calculated in the same way?

No. Grades for beach sites are ‘all weather’ grades meaning they are based on all routine water sample results from the past five summers, whereas the grades for river sites are based on ‘dry weather’ conditions. This means that we only use the results from monitoring undertaken during low or moderate river flow conditions to calculate the grade (ie, when swimming and other types of recreation are most common). See our latest annual monitoring report for more information on how grades are calculated as well as ‘all weather’ grades for rivers.

Why is there a difference between the grades on your map and the grades shown on LAWA?

Land Air Water Aotearoa (LAWA) is a collaboration between NZ’s regional and unitary councils, Cawthron Institute, Massey University and Ministry for the Environment which has resulted in the establishment of a website with one aim: To tell the story of our environment.

For some sites, there is a difference in the grade shown on our interactive water quality map and the overall recreational risk shown on LAWA.

At coastal sites such as Balaena Bay, Shark Bay and Plimmerton Beach at Bath St this is because different time periods are used to the calculate grade or recreational risk. Grades shown on our site are calculated using the last five years of data, whereas LAWA has only used the last three years of data.

At river sites the differences between grades due to the grades being based on data collected during ‘dry weather’ conditions only (see above). The overall recreation risk on LAWA is based on data from all samples collected over the last three years including those collected during or shortly after heavy rain.

The good, the bad and the ugly: The causes of poor water quality 

Where are the best places to swim?

The best sites are generally in the upper reaches of rivers that are in forested catchments, and at beaches that are away from streams or stormwater pipes draining urban or agricultural catchments. Click here to see grades for river and beach sites across the region.

What causes poor water quality? 

The cause of poor water quality depends on the land use in the surrounding area or upstream catchment. In rural areas the most common source of pollution is agricultural land use. In wet weather, excess rainwater flows over farms and into nearby streams and rivers, picking up manure and other contaminants along the way. In dry weather, stock defecating directly into streams or rivers can cause contamination of swimming spots, and septic tanks can also be a source of contamination in rural areas.

In urban areas stormwater and sewer leaks or overflows are the main sources of contamination. During wet weather, rainwater from roofs, roads, car parks and other surfaces is piped directly into rivers, streams and the coast. During its travels this stormwater picks up sediment, rubbish, animal faeces and other contaminants. During very heavy or prolonged rainfall, sewage overflows can result in untreated sewage being discharged to rivers or the coast via the stormwater system.

During stormy weather conditions, heavy rain and wind can also churn up sediment from the bottom of the waterway or sea, releasing pathogens in the sediment back into the water.

During dry weather, cracked or blocked sewer pipes as well as illegal connections of private sewers to the stormwater system can also cause contamination of our waterways. Large numbers of birdlife such as ducks, geese and seagulls can also contribute to faecal contamination, especially in dry weather.

What effects do treated sewage discharges have on swimming water quality?

It is difficult to assess the effect of treated sewage discharges on water quality as the treatment process often removes faecal indicator bacteria such as E. coli but may not remove the pathogens that can make people sick (such as viruses). Unless tests have been undertaken to show how effective a sewage treatment plant is at removing all pathogens, we consider it appropriate to give potentially affected sites a more conservative grade. For example, sites on the Ruamahanga River that are downstream of discharges from the Masterton, Carterton, Greytown and Martinborough sewage treatment plants are graded C or D depending on how far away they are from the discharges. This is despite these sites often having low E. coli counts during dry weather.