Te Whatu Ora last week issued a warning about toxic blue-green algae in Lake Hood near Ashburton.
National Public Health Service specialist Dr Claire Salter said the person was exposed to the bacteria before the warning was issued.
Symptoms might not appear for quite some time until after contact with the water, she said.
"Toxins released by cyanobacteria can trigger asthma and hayfever symptoms, skin rashes, stomach upsets, and neurological effects such as tingling around the mouth, headaches, breathing difficulties and visual problems."
The lake was tested weekly by Canterbury Regional Council but concentrations could change due to wind or warm weather.
The council's water and land science manager Dr Elaine Moriarty said the bacteria was commonly found in Canterbury's lakes, especially during summer and autumn.
