A health warning has been issued for a fifth lake in north Waikato, following the discovery of high levels of toxin.
Lake Waahi, near Huntly, along with Lakes Ngaroto, Whangape, Hakanoa and Waikare has cyanobacterial blooms, which pose health risks including skin irritations and possible stomach upsets.
Lake Rotoroa in Hamilton also has high levels of the bacteria.
Waikato District Health Board says the risk to the public has increased significantly because high winds have caused cells to concentrate and form scum around lake edges.
It says people should avoid making skin contact with the water during the blooms.
If contact does occur, people should shower and change their clothing immediately.
Medical Officer of Health Dell Hood says the blooms are common in late summer.