Warning signs will be put up along the Twin Coast Cycle Trail warning users not to drink from or swim in the Utakura River after a potentially toxic algal bloom upstream in Lake Omapere.

The cycle trail's most scenic section follows the Utakura Valley from Okaihau to the Hokianga Harbour, with riders often taking a dip in the river to cool off.

However, an algal bloom in the lake which reached a ''tipping point'' on February 16 has turned the river bright green and killed many of its eels. Lab tests to determine whether the bloom is toxic are expected to be completed early next week.

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Some locals are unhappy about the lack of warning, which seems to stem from unclear and overlapping roles of the three agencies involved — the Northland Regional Council, Far North District Council and Northland District Health Board.

District council spokesman Ken Lewis said staff were working with the lead agency, the regional council, to establish the level of risk to people and livestock from the algal bloom.

As a precaution the district council was planning to erect signs along the cycle trail warning users not to drink from or bathe in the Utakura River.

It was not clear, however, when the signs would go up.

Meanwhile, the regional council is defending its handling of the bloom, with some water users downstream unhappy they weren't warned levels of cyanobacteria — the organism causing the bloom — had reached a ''trigger point'' three weeks earlier.

Utakura Valley farmers Neville and Linda Lewis, who use the water for drinking and stock, said they found out only when the river turned bright green and was capped with foul-smelling foam. They had not received any official warning.

A thick layer of foam on the Utakura River has dissipated but the water remains an intense green. Photo / Linda Lewis
A thick layer of foam on the Utakura River has dissipated but the water remains an intense green. Photo / Linda Lewis

Regulatory services manager Colin Dall said it was the regional council's job to keep the district council and district health board informed about the results of water quality tests, not to issue health warnings or put up signs.

However, a staff member inspecting the bloom on Saturday visited people living near the lake and in the valley to inform them. He also asked them to warn their neighbours.

Speaking to a regional council meeting on Tuesday, Mr Dall said several factors contributed to the bloom reaching a tipping point, including nutrient run-off into the lake, high temperatures and rainfall, and easterly winds stirring up sediment and pushing algae towards the outlet.

The eel deaths were likely caused by the algae using up the oxygen in the water.

The overlapping roles of the councils and health board when it came to water monitoring and warning the public highlighted the need for the three organisations to finalise a protocol for dealing with blooms. Mr Dall hoped it would be completed next week

The health board is waiting for the outcome of toxin tests before deciding if a specific warning is needed. In the meantime it repeated its standard warning about not gathering shellfish or swimming after heavy rain, especially in fresh water.

While few people swim or fish in Lake Omapere itself, the Utakura River is used for swimming and as a water source, and people gather shellfish near its outflow in the upper Hokianga Harbour.

The lake level has been lowered a number of times since 1905 to create more land for farming. Though Northland's largest lake it is just 1.8m at its deepest point, making it vulnerable to blooms.

A severe bloom in the 1980s killed most life in the lake; a smaller bloom followed in 2003-04.

Work since then to improve water quality and reduce farm runoff has included wetland restoration and riparian planting.