The warning covers a section of the river near State Highway 7.
Medical Officer of Health Dr Cheryl Brunton said people should avoid the area.
All animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
Potentially toxic algae may also be found in other access points along the river near SH7.
Brunton advised people to check for the algae at every low-flowing river in the region and avoid contact with the water.
Brunton said the algae has 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, seek medical advice urgently and let your healthcare provider know you’ve had contact with dark brown/black algal mats or water in this area." Brunton 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 waterways until the warnings have been lifted.
Environment Canterbury is monitoring the sites and the public will be advised of any changes in water quality.
What is potentially toxic algae (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 councils may place warning signs, they may not be seen at the numerous river access points.
