A Northland lake and river have tested negative for toxins but people are still being warned not to swim there.

An algal bloom at Lake Omapere and the Utakura River has turned the water bright blue and green, left white foam on top and probably killed some eels.

Northland Regional Council says there are no cyanotoxins in the water but there are high levels of the bacteria that can produce them.

Spokesman Colin Dall said people still shouldn't swim in the water or use it for household or stock supply.

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The council would continue to monitor the lake and river until the algal bloom receded, which could be some time away, he said.