Continuing warm dry weather in Canterbury is prompting further warnings about rivers developing potentially toxic algal growths.

Extensive growth of the cyanobacterium (blue-green algae) Phormidium has been observed in North Canterbury's Ashley River/Rakahuri and South Canterbury's Pareora River and downstream reaches of the Opihi River in recent days.

The Medical Officer of Health, Dr Ramon Pink, advises that because of the increased levels of algal growth, the water in these rivers is unsafe for recreational use. Both human and animal contact with the water, and with the dark brown or black Phormidium mats growing on the river bed, should be avoided. Ingestion of toxic algae, or water containing algal toxins, can be fatal to animals, including dogs, sheep, cattle, fish and birds. All river water is potentially unsafe to drink direct from the river, and the Medical Officer of Health recommends that people consume water only from suitable sources.

Swimmers have been known to develop adverse reactions after bathing and showering in water containing blue-green algal blooms. These include allergic reactions, asthma, eye irritations, rashes and blistering around the mouth and nose and gastrointestinal disorders (abdominal pain, cramps and diarrhoea).

Warning signs advising people of these risks will be placed around the rivers.

In addition to the rivers named above Environment Canterbury is issuing a general warning to river users throughout the region that Phormidium is likely to be present in some river areas. Dog owners should be particularly wary, and keep their dogs away from any areas of rivers where dark brown/black algal mats are present. People should not swim in areas where there are large amounts of dark brown/black algae.

The toxin-forming, naturally-occurring black algae, Phormidium, is found in rivers throughout the country, but only forms problematic thick blooms at some sites under the right flow conditions. In thick blooms the dark mats can peel off the substrate and show a lighter coloured underside. Although district or city councils may place signs warning of Phormidium, these may not be seen at the numerous river access points, hence the need for people to treat every low-flowing river cautiously.

Dog owners should keep their animals on a leash in any areas where they can see the blackish/brown algal mats. Please report any sightings and locations to the regional council in Timaru or Christchurch.

For further information or in the case of illness or suspected illness, seek advice from your doctor. If animals become sick, contact your veterinarian.