The Otago Regional Council has issued another health warning for Lake Waihola, near Milton, just over a month after lifting its previous warning.
High concentrations of cyano-bacteria - or blue-green algae - have once again been discovered in the water.
The bacteria is harmful to humans and animals, and can cause rashes, sickness, and tingling and numbness.
It flourishes when nutrients in the lake bed are stirred up by strong winds.
People should avoid swimming or wading in the lake, and keep animals out of it.
Anyone who comes into contact with cyano-bacteria should visit their doctor if they show symptoms.