Another lake in the Far North has succumbed to algal bloom, with health warning signs going up at Lake Waiparera, north of Kaitaia.
The 100-hectare lake is the largest of the fragile dune lakes on the Aupouri Peninsula.
It was once a popular spot for camping, waterskiing and sailing, but official warning signs now advise visitors they enter the water at their own risk.
Northland Environmental Protection Society president Fiona Furrell said the bloom was entirely preventable.
"Cattle graze both the DoC reserve and the council strip margin surrounding the lake, and cattle sunbathe themselves and cool off in the lake regularly. So if cattle defecate in a perched lake, that's exactly where it stays, because there is no river to flush that lake."
Ms Furrell said the Department of Conservation and the Far North District Council never bothered to fence the land around the lake.
Algal bloom has also polluted Lake Omapere near Kaikohe and the nearby Utakura River.