Signs are up at one of Whanganui's safest and most gentle swim spots - Lake Wiritoa - warning people not to touch or drink the water.
They follow signs put up at Dudding Lake in recent weeks, and permanent health warning signs put up at three coastal streams north of Whanganui - the Mōwhānau, the Kai Iwi and the Ototoka.
The two lakes and the streams have been popular spots for swimming - especially for children, who are more at-risk from poor water quality.
Health-risk signs are now up at Lake Wiritoa because the level of blue-green algae has climbed to twice the maximum level recommended for swimming.
The algae is not toxic, but at this density it can cause skin rashes, nausea, stomach upset and tingling and numbness around the mouth or finger tips.
Anyone experiencing symptoms is advised to see a doctor, and tell the doctor they have been in contact with the lake water.
It could be harmful to humans and animals if swallowed, or contacted through swimming, kayaking or water skiing.
Boiling the water will not prevent it being potentially harmful, Whanganui District Health Board says.
The lake is sometimes stocked with trout but fish can concentrate any toxins, and should be avoided. If people wish to eat them, their guts and liver should be removed and they should be washed in clean water first.