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  • Why this status?

    Click here for detailed information on water quality monitoring

    The science

    Summer season monitoring

    LAWA shows the best available water quality information to help you decide where to swim.

    Special Status

    Special StatusIssued: 21 Nov 2023

    Unsuitable for swimming.

    This site has temporarily been determined as unsuitable for swimming, and we recommend people do not swim at this site.

    River warnings are now in place for toxic algae (benthic cyanobacteria) in the:

    • Hae Hae Te Moana River at Te Moana Gorge 
    • Waihao River at Bradshaw Bridge
    • Pareora River/Pureora at Pareora Huts
    • Opihi River at State Highway 1 Bridge
    • Ashley River/Rakahuri at State Highway 1 Bridge
    • Selwyn River at Glentunnel


    The warning follows finding moderate to high cover of toxic algae (benthic cyanobacteria) at the above locations.

    People should avoid the area and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
    There are also other access points along the affected rivers that may have benthic cyanobacteria present. People are advised to treat every low-flowing river cautiously, check for the presence of benthic cyanobacteria and avoid contact.

    Dr Matthew Reid, Medical Officer of Health for Te Mana Ora, National Public Health Service, says the algae look like dark brown to black mats and can produce toxins harmful to people and animals.

    “Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.”

    “If you experience any of these symptoms, visit your doctor immediately, also let your doctor know if you’ve had contact with dark brown/black algal mats or water in this area”, Dr Reid says.

    Pets that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.

    People and animals should remain out of the waterway until the warning has been lifted.

    Environment Canterbury continues to monitor popular swimming sites and the public will be advised of any changes in water quality.

    Facts about benthic cyanobacteria:
    • Appears as dark brown/black mats attached to rocks along the riverbed.
    • A low cover of the algae can occur naturally but can increase rapidly during warmer months.
    Algal blooms are influenced by a combination of available nutrients in the water and sediments (such as nitrogen and phosphorus), a sustained period of low and stable flows, and favourable weather conditions (e.g. increased temperature, calm days).
    • It often has a strong musty smell and algal toxin concentrations can vary over short periods.
    • Although high river levels will remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins.
    • If a health warning is in place avoid contact with the water.
    • Although district or city councils may place warning signs, these may not be seen at the numerous river access points, hence the need for people/ dog-walkers to treat every low-flowing river cautiously.

    For more information on what toxic algae is and what to look out for, visit:



    Heavy rain flushes contaminants from urban and rural land into waterways and we advise you not to swim for 2 – 3 days after heavy or prolonged rain – even at sites that generally have good water quality. Check that the water is clean and clear before taking a dip.

    Potentially toxic algae can rapidly bloom to harmful levels, and not all freshwater sites are monitored for toxic algae. Play it safe — if you can see toxic algal blooms in rivers or lakes, avoid contact or choose another site to swim.

    Summer season monitoring history

    • Samples this season
    • Samples from last 5 years
    E. coli sampling graph
    • View as graph
    • View as donut
    Toxic Algae sampling graph

    What do the monitoring results mean?

    Suitable for swimming – The monitoring result met national water quality guidelines at the time of testing.
    Caution advised – The monitoring result for E. coli was slightly elevated at the time of testing. Water quality generally suitable for swimming, but young children, elderly or those with compromised health may be at increased risk of illness.
    For sites where potentially toxic algae is routinely monitored, the monitoring result exceeded the surveillance criteria. Avoid contact where potentially toxic algae is present.
    Unsuitable for swimming – The monitoring result did not meet the national guidelines at the time of testing.
    For sites where potentially toxic algae is routinely monitored, the monitoring result exceeded the alert level criteria. Avoid contact with the water and with potentially toxic algae along the edge of the water.

    Long-term grade information

    Long-term grade based on five years of data
    Minimum data points required 50 Highest E. coli per 100ml value {{riskData.maxValue | number:0}}
    Number of data points available {{riskData.dataPoints | number:0}} Lowest E. coli per 100ml value {{riskData.minValue | number:0}}
    Hazen 95th percentile result {{riskData.hazenResult}} Long-term grade

    What do the long-term grade icons mean?

    Excellent – The estimated risk of Campylobacter infection has a <0.1% occurrence, 95% of the time.
    95th percentile of E. coli / 100 mL is <= 130.
    Good – The estimated risk of Campylobacter infection has a 0.1 - 1% occurrence, 95% of the time.
    95th percentile of E. coli / 100 mL is > 130 and <= 260.
    Fair – The estimated risk of Campylobacter infection has a 1 - 5% occurrence, 95% of the time.
    95th percentile of E. coli / 100 mL is > 260 and <= 540.
    National bottom line: 95th percentile: 540
    Poor – The estimated risk of Campylobacter infection has a >5% occurrence, at least 5% of the time.
    95th percentile of E. coli / 100 mL is > 540.
    Not available – There were not enough data to determine the long-term grade at this site.
  • What about my dog?

    Information about toxic algae

    Dog owners

    Keeping your dog safe and healthy this summer

    A trip to the river is a lot of fun and knowing how to keep your dog safe means you are able to relax and enjoy the adventure even more. Potentially toxic algae can be harmful to dogs (and us) – this section shows you what to look for.

    Toxic algae from Cawthron Institute on Vimeo.


    What to do

    •  The best thing you can do to keep yourself, your kids and your dog safe is to know what potentially toxic algae looks like and avoid it. 

    • If you are not sure, keep your dog on a lead at a stony river, and don’t let them in or near the water.  Bring water from home for your dog so that they don’t need to drink out of the river.

    What is potentially toxic algae?

    Freshwater algae are naturally occurring in rivers and lakes in New Zealand, including waterways with good water quality. Most algae is harmless, if a bit slimy – but one group of algae, known scientifically as cyanobacteria, can be toxic.  In rivers, this potentially toxic algae is also known as Phormidium.  Most of the year it is present at low levels that are not much of a danger.  But it can be a problem during the summer months, when low rainfall, warm temperatures and more sunlight create an environment where it can thrive.  In parts of stony-based rivers it can form extensive mats which can be toxic to humans, dogs, livestock and wildlife. 

    Scientists do not yet understand when and why this algae turns toxic, so to be safe, always treat it as toxic.

    Dogs are particularly susceptible to poisoning from toxic algae as they love to scavenge and play near water. 

    What to look for

    In parts of stony based rivers toxic algae form soft, leathery-looking mats on rocks in the riverbed.  These mats range in colour  from blackish/brown to dark green (it's quite different from normal harmless green algae, which looks bright green and often forms long strings).

    These mats can come loose and wash up on the edge of the rivers, or form ‘floating rafts’ in shallow areas.  As they dry out they turn light brown or white and look like dried leaves or cowpats. They produce a strong musty smell, and this is when it poses the biggest risk to our dogs, as they love the smell and many dogs will try to eat it if they get the chance.

    Council staff monitor for these mats in parts of rivers where these are known to form at popular swim sites.  Staff are not able to monitor all parts of rivers, so please learn what to look for to keep your dog safe, and alert your regional or unitary council when you see algal mats so they can warn other people.

    What if my dog has eaten potentially toxic algae?

    If you suspect that your dog has eaten toxic algae, you should contact your vet as soon as possible.  Signs a dog has been poisoned by toxic algae include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsions.

    Find out more about potentially toxic algae here

  • What's at this site?

    Find out all recreational activities and facilities at this site

    Activities & facilities

    Site suitable for these activities and has these facilities

    Our lakes, rivers, and beaches are great natural playgrounds but they can be unpredictable.  Be aware of other potential risks such as rips, strong currents, sudden drop offs, or underwater objects before jumping in.  LAWA recommends that you avoid swimming for 2 - 3 days after heavy rainfall and follow the advice of any warning signs in place.


    Dog walking

    To find out when and where you can exercise your dog please refer to your local authority.

    View exercise areas

    Check the rules


    Picnic area

How do I get there?

Waihao River