The Northland Regional Council knew a toxic algal bloom was likely at a Far North lake and failed to warn the community, a local farmer says.
Linda Lewis, who farms near Lake Omapere. said the massive bloom that appeared on the lake on Saturday has turned its outlet, the Utakura River, into a toxic soup.
Water temperatures in the shallow lake were 25°C last week and the regional council knew from its tests that a bloom was likely, Mrs Lewis said.
It gave no warning to farmers who draw water from the river, people downstream who swim in it, or tourists on the nearby cycle trail, she said.
"I think the council needs to front up and say yes, they did know about it and perhaps apologise to all the people down the Utakura River and Hokianga and do something about putting some signs up."
Mrs Lewis and her husband have now put up their own signs on the cycle trail, to warn people not to drink the water or bathe in it.
The couple managed to switched their water supply from the river in time to avoid poisoning their stock, only because a neighbour warned the lake didn't look right, she said.
The next day, it had 'flipped' and the river was running green with a blanket of white foam caused by the algal bloom.
Local farmers, tangata whenua and the Lake Omapere Trust had worked hard over many years to improve the lake, planting its margins with native species, Mrs Lewis said.
"It's really sad to see this happen - there's masses of dead eels killed by the algae and I guess part of it is climate change, but we all have to work together and the council's communication has been really poor," she said.
Linda Lewis said the last bad algal bloom on Lake Omapere was back in the 1980's but there had been cases of children at Horeke with skin rashes after swimming in recent weeks.
The Northland Regional Council has not yet responded to a request for comment.