Independent tests have found no toxins in Northland's Lake Ōmāpere and the Utakura River, after a massive algal bloom.
But the Regional Council said people should still stay out of the water.
The algal bloom in the shallow lake near Kaikohe erupted last weekend, turning the river green, and killing hundreds of eels.
The council's regulatory manager Colin Dall said test results just received detected no cyanotoxins, which cause health problems.
But he said the bacteria that can produce the toxins are present at very high levels and people should not swim in the lake or river or use the water for their stock or households.
The bloom on the Utakura River was capped with a foul-smelling white foam, which some locals said burnt their lips if they got close to it.
Local farmers, tangata whenua and the Lake Ōmāpere Trust have been trying to improve the lake, planting its margins with native species.
The lake has seemed healthy in recent years, but in the 1980s and again in 2003, toxic algae blooms have polluted the Utukara, making the water unusable for some time, and on at least one occasion, poisoning stock.
The Far North District Council is erecting signs on the nearby Twin Coast cycleway, warning users to keep out of the water.