The Canterbury District Health Board's community and public health unit issued a health warning for the Ashley/Rakahuri River at State Highway 1 (SH1) on Monday.
Testing showed high levels of blue-green algae (benthic cyanobacteria) upstream of the SH1 Bridge.
The same toxic algae was found in the river earlier this month, further west at the Cones Rd bridge between Rangiora and Loburn.
People should avoid the area of the Ashley/Rakahuri River and animals, particularly dogs should not be allowed near the water until the health warning has been lifted, a statement said.
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A health warning has been issued for the length of the Canterbury's Ashley/Rakahuri River after more toxic algae was found.
The Canterbury District Health Board (CDHB) says moderate to high cover of potentially toxic algae (benthic cyanobacteria) has been found along the river between the Okuku River Confluence to the State Highway 1 bridge.
A warning was already in place for the SH1 site, and while a warning was recently lifted for the Loburn-Rangiora site because it had become dry, new algae growth was already being seen there.
Canterbury medical officer of health Dr Ramon Pink says the algae looks like dark brown to black mats and can product toxins harmful to people and animals.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips."
Anyone who experiences such symptoms should visit their doctor immediately, and animals showing signs of illness should be taken to a vet immediately.
Reticulated town water supplies are currently safe, but no-one should drink water from the river at any time, Pink said.
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