The Community and Public Health division of the Canterbury District Health Board have issued a health warning after potentially toxic algae (benthic cyanobacteria) were found in the Ashburton, Waimakariri and Ashley Rivers.

Canterbury Medical Officer of Health Dr Ramon Pink says the algae look like dark brown/ black mats and can produce toxins harmful to humans and animals.

"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips," Dr Pink says.

"If you experience any of these symptoms, visit your doctor immediately and let your doctor know if you've had contact with algal mats or water in this area.

"No one should drink the water from the river at any time."

Boiling the water from the river does not remove the toxin. However, reticulated town water supplies are currently safe.

Animals should be taken to a vet immediately if they come into contact with the contaminated rivers, Dr Pink says.

Humans and animals, particularly dogs, should avoid the following sites until the health warning has been lifted:

? Ashburton River at State Highway 1 bridge ? Ashley River at State Highway 1

? Waimakariri River at Reids Reserve

Environment Canterbury is monitoring the sites and the public will be advised of any changes in water quality.

Facts about cyanobacteria:

- Appears as dark brown/black mats attached to rocks along the riverbed.

- The algae occur naturally but can increase rapidly during warmer months.

- It often has a strong musty smell and algal toxin concentrations can vary over short periods with changing environmental conditions.

- 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.

All current warnings remain in place. For further information visit http://ecan.govt.nz/services/online-services/monitoring/swimming-water-quality/Pages/river-warnings.aspx

Or contact Community and Public Health on (03) 364 1777.