A lake known as 'the food basket of the North' to local iwi is in crisis as an algae bloom sets in.
The water of Lake Ōmāpere and the nearby Utakura River, which flows out to the Hokiānga Harbour, has turned highlighter green, topped with thick white foam and a foul stench.
Local farmer Neville Lewis says it's killing eels.
"I've seen a couple of dead ones and some swimming around trying to get air, because I guess there's no oxygen in that water now."
NIWA and Ngāpuhi teamed up to restore the health of Lake Ōmāpere in 2008. The lake is highly significant to the iwi, and particularly hapū in the Kaikōhe area.
Mr Lewis said he hasn't seen algae bloom this bad in decades.
"In the early-mid '80s it happened, but I think this is worse. It had been changing colour for the last couple of days but yesterday afternoon it was dramatic, almost scary to be fair.
The Twin Coast cycle trail goes from Hokiānga Harbour to the Bay of Islands and passes Lake Ōmāpere. Mr Lewis fears tourists using it will be less than impressed.
"It's a disaster for the Hokiānga because people come down here, our part of the cycleway is pretty spectacular, everybody loves it, but this is not a pretty sight. Fingers crossed it's going to clear itself quickly, but I don't think it will."
Mr Lewis said it took years for the water to become drinkable again for people and animals when the last incident happened in the 1980s.
The Northland Regional Council (NRC) is warning people and animals to stay away from the water while it conducts tests.
“Until we've got the samples back we don’t know what kind of algae it is and we don't know the level of toxins in the lake," said Tess Dacre, NRC compliance monitoring manager.
"We know blooms result from high nutrients and we've had a lot of rainfall so that would've contributed - and then the warm weather is obviously a factor."
The NRC believes cyanobacteria blooms are more likely to occur due to climate change.
It is working on a framework, which is still in a draft phase, to combat the issue in conjunction with the local district councils and the Northland District Health Board.