Dog owners in some regions are being warned to watch out for toxic algae in rivers and streams.

In Marlborough, council staff have spotted high concentrations of algae in parts of the Omaka and Wairau rivers.

Warning signs have been placed at some access points, but not all sites are monitored.

People walking their dogs should be alert for any riverbeds covered in a thick brown, or black, mat with a velvety appearance.

Dogs are attracted to algae's musty, earthy smell and but can die if they eat it.

People, especially children, should avoid touching or swimming near algae, as it can cause vomiting and skin irritation.