Swimmers are being told to avoid Lake Hayes, near Queenstown, due to high levels of E coli.
Recent testing by the Otago Regional Council has found that the lake, which is between Queenstown and Arrowtown, has almost double the E coli levels that are deemed safe.
Council spokesperson Dean Olsen said the level of bacteria could be influenced by many factors, including rainfall, water flow, bird activity, stock in waterways or even the weather.
Further testing was underway, but in the meantime swimmers should stay away from Lake Hayes, he said.
The test results are expected to be released by Wednesday.