The Southland Regional Council is warning duck shooters to avoid using Waituna Lagoon because of high levels of toxic algae.
Environment Southland has detected high levels of planktonic cyanobacteria in the lagoon.
This type of algae can appear as suspended green globules, a greenish tinge, or may be invisible to the human eye.
Just because it could not always be seen did not mean it was not there, council director of science and information Graham Sevicke-Jones said.
With the duck shooting season opening this weekend, he said people and animals should avoid contact with the lagoon's water.
The algae is likely to cause skin rashes and blisters, but stomach pain and nausea are also possible.
Anyone experiencing symptoms should visit a doctor immediately.
Dogs who consume the bacteria will also need immediate medical attention.