Toxic algae

Why toxic algae is dangerous

Even small amounts of toxic algae (about the size of a 50-cent piece) can cause serious illness if swallowed. Toxic algae can pose health risks through:

swimming and water activities
drinking contaminated water
gathering shellfish and mahinga kai.

Risks to people

Health effects may include:

tingling or numbness around the fingertips and/or mouth
breathing difficulties
gastrointestinal symptoms
skin rashes.

Watch video: Toxic algae in our rivers – What New Zealanders need to know

Risks to dogs and livestock

It can be especially dangerous for dogs as they love the musty smell it produces and are naturally drawn to investigating it – usually by licking or ingesting the material when out enjoying rivers and lakes across the country.

If your dog is showing symptoms after being in contact with a waterway, contact a veterinarian immediately. Common symptoms include:

panting
lethargy
muscle tremors
twitching
convulsions.

If your animal becomes unwell after contact with a waterway, contact a veterinarian immediately and report it to Environment Canterbury.

Our monitoring may not detect the small amounts of toxins that can make dogs sick, so pets should be kept away from any signs of algae.

What causes toxic algae

Toxic algae is found in rivers, lakes and beaches across Aotearoa, including Waitaha Canterbury.
Blooms happen when the algae multiply rapidly, usually due to changes in environmental conditions like warm temperatures, abundant sunlight, low or stable river flows, and high nutrient levels.
These conditions often occur during hot, dry periods or after particularly wet seasons.
The nutrients that fuel blooms, mainly nitrogen and phosphorus, come from natural sources and land use (urban and rural). Common contributors include wastewater and sewage, stormwater runoff, and agricultural runoff.

Where nutrients come from

The nutrients that trigger these blooms, primarily nitrogen and phosphorus, come from both natural sources and human activities.
Common nutrient contributors include:
- Wastewater and sewage
- Stormwater run-off carrying contaminants from our urban centres like lawn fertilisers, sediment, and heavy metals from our vehicles
- Agricultural run-off from livestock and fertilisers

Toxic algae in rivers: What it looks like

To help keep you and your pets safe, check for signs of toxic algae before going into rivers. Below are descriptions of what toxic algae in rivers can look like.

What it looks like in rivers

Grows on the bottom of riverbeds: Toxic algae (benthic cyanobacteria) attaches to rocks and gravel on the riverbed.
Forms dark, slimy mats: It appears as thick dark brown or black mats that have a slimy or velvety texture and musty smell.
Can break loose and float: Mats can come loose and wash up on the edge of the rivers, or form ‘floating rafts’ in shallow areas.
Changes appearance as it dries: As they dry out, they turn light brown or white and look like dried leaves or cowpats.

What it looks like in lakes, ponds and lagoons

Suspended in the water: Floating toxic algae (Planktonic cyanobacteria) are often mixed through the water rather than attached to the riverbed.
Can change the colour or clarity of water: When it blooms, it can make water look cloudy, discoloured, or like it has small globules in it.
May be hard to see: In some cases, there are no obvious visual signs, even when toxic algae is present.

Health warnings: Before you head out, check current health warnings for rivers and lakes to keep people and pets safe.

Check before you swim

We monitor water quality at many of the region’s recreational swim sites between November and March. However, we cannot monitor all sites.

Water quality results on Land Air Water Aotearoa (LAWA) and health warnings are based on the risk to human health, not animals.

Check water quality before you go

Caution advised at these rivers in Waitaha Canterbury

Toxic algae (cyanobacteria) are known to grow in many of our hill-fed and some alpine rivers and may pose a risk to humans and dogs. Please be cautious and ensure you check any river for toxic algae before jumping in.

These rivers are known to have toxic algae blooms at times:

Hae Hae Te Moana River
Leader River
Opihi River
Ōtākaro Avon River
Otaio River
Pareora River
Rakahuri Ashley River
Temuka River
Waiau River
Waihao River
Waihi River
Waikirikiri Selwyn River
Waimakariri River
Waipara River
Waitohi River

Monitored swimming spots along these rivers may not have an active public health warning. This is because the amount of toxic algae could be below the threshold for a public health warning.

What’s happening this season?

Conditions this season are similar to previous years, but you may see more health warnings due to improved and faster communication to help communities stay informed and safe.

Is the problem getting worse?

In Waitaha Canterbury, water quality remains relatively stable.

80% of our monitored swimming sites suitable for recreation.
We’ve been monitoring for toxic algae at recreational sites since 2009.
Year-on-year, this season is similar to previous years in terms of temporary health warnings.
In December 2025 there were 10 active temporary health warnings, comparable to:
- 9 warnings in December 2024
- 8 warnings the year before.

Why are there so many health warnings this season?

In recent years, we’ve changed how we communicate about temporary health warnings for toxic algae and faecal contamination.

Our website and social media allow us to share warnings faster, often before signage appears on site.
This helps communities identify affected sites and recognise blooms earlier.
In addition to signage, we share updates on our Facebook page using information from the National Public Health Service.

Find out more

Read the factsheet on toxic algae on the Land Air Water Aotearoa website
Download our toxic algae brochure (PDF File, 745.85KB)
Check for current health warnings in lakes and rivers.