Swimmers are being advised to stay away from lakes in the Waikato region that are affected by algal blooms.
The Waikato District Health Board says health warnings are in place for Lakes Ngaroto, Hakanoa, Waikare, Waahi and Whangape.
The DHB says these lakes have tested positive for cyanobacteria, which can cause rashes, allergy symptoms and stomach upsets.
Medical officer of health Dr Dell Hood says people should avoid touching or swallowing the water and especially any scum from the lakes.
If people still choose to use the lakes, Dr Hood says they should shower and change their clothing as soon as possible afterwards.