A "very high" level of bacteria, but no toxins, has been found at a Northland lake that's turned bright green and smelly.
The algae bloom in Lake Ōmāpere and the nearby Utakura River has killed hundreds of eels and freshwater fish.
Northland Regional Council (NRC) group manager for regulatory services Colin Dall says they were likely suffocated, not poisoned.
"One of the effects of the bloom is that the algae use oxygen in the water and that means that oxygen isn't available for eels or other fish. They basically suffocate and die."
Mr Dall says NRC will be regularly testing the lake and surrounding waterways as the bacteria is capable of producing cyanotoxins.
"You can't predict when these bacteria do produce toxins; it can change at any time."
He is urging those downstream in the Hokiānga Harbour to avoid collecting seafood while the algae bloom remains.
"Shellfish filter water, so they will be filtering in some of the bacteria and potentially toxins. At this stage we haven't detected toxins in the water, but it's better to be safe than sorry."
Forest and Bird says NRC needs to better manage land use around the lake to prevent pollution from entering it.
"Often times high nitrate levels are associated with these kinds of algae blooms," says freshwater advocate Annabeth Cohen.
"We need to really make sure what we're doing on land isn't affecting our freshwater fish and eels."
Ms Cohen says 74 percent of New Zealand's freshwater fish and eels are at risk or threatened with extinction.