Toxic algae

Common name:  Toxic algae, cyanobacteria, blue-green algae
Scientific name:  Various species 
Management programme:  Monitored


Why is it a pest?

Toxic algae, or cyanobacteria, naturally occur in rivers and lakes throughout Otago. Toxic algae can grow rapidly and form ‘blooms’, most commonly during the summer months. Toxins produced by these blooms can be harmful to human and animal health.

What does it look like?

Toxic algae in rivers

Toxic algae generally form brown or black mats that grow on the river bed. Mats may come loose and float downstream, wash up on banks, or become caught up with other river debris. When the mats die and dry out, they become light brown or white.

The more brightly coloured long green algae commonly found in rivers and streams are harmless algae that don't produce toxins. 

Toxic algae in lakes

In lakes, ponds and slow-flowing waters, toxic algae grow if a free-floating form which give the water murky or cloudy appearance, like green pea soup. It can be visible as small globules suspended in the water or form scums on water’s surface, especially at the water’s edge. While blooms are generally green, they can vary in colour from red to bright blue.

Toxic algae in rivers: what you need to know

Toxic algae frequently asked questions

What are the health risks for people and animals?

How can I keep safe?

About toxic algae

Seen a suspected toxic algae bloom?

Report suspected toxic algal blooms to the ORC Pollution Hotline 0800 800 033 (7 days) or email us on

Photographs of observed algal blooms will help us to quickly identify toxic algae and notify others about potential risks.

Management programme


Page last updated 26 June 2024.