Holes visible in the Ōhau Channel diversion wall. Photo / Supplied
Rotorua’s mayor has taken aim at the regional council as she joins a chorus of voices calling for an urgent fix for a corroded lake wall.
Two community groups have raised alarm bells over the state of the Ōhau channel diversion wall, which was built in 2008 to stop more polluted waters from flowing into Lake Rotoiti.
The groups presented an urgent letter of demand to the Bay of Plenty Regional Council over their concerns for the lake.
But the regional council insists it is not breaching consent over the wall, which they say remains structurally sound despite unforeseen microbial corrosion.
The $11 million wall was built to stop nutrient-rich polluting water from Lake Rotorua spilling into Lake Rotoiti and degrading its quality.
It was expected to last 50 years, but images show significant corrosion.
The letter and supporting documents, sent by the Lakes Water Quality Society and Lake Rotoiti Community Association, were delivered to the regional council on Monday morning.
It claims the regional council is currently operating in breach of its resource consent and highlights the presence of several corrosive holes in the wall.
The groups suggest nitrogen levels are rising, water clarity is declining, algal blooms are increasing, and the regional council has ignored warnings since 2014.
The letter was supported by local businesses including Hell’s Gate, Lake Rotoiti Hot Springs and Pure Cruise, as well as representatives from Ngāti Pikiao.
Rotorua Mayor Tania Tapsell also criticised the regional council for its inaction, backing the letter and sharing surprise at the extent of the corrosion.
“Continuing to delay action to resolve this is not an option and I support the urgent call for Bay of Plenty Regional Council, as owners of the wall, to fix it as soon as possible,” said Tapsell.
While the Rotorua Lakes Council was a partner in the wall build, the regional council is the legal owner and is responsible for maintenance and repair.
Lake Rotoiti Hot Pools owner Tim Manning said the wall had worked, but inferior materials and a ‘head-in-sand’ approach from the regional council led to the lake’s deterioration.
“It’s failing and the lake is getting worse,” said Manning.
“Rotoiti is the jewel in New Zealand’s crown. Yet the council are letting residents, tourists, and businesses down. They must front up, admit it’s their problem, and fix it before the community suffers.”
Bay of Plenty Regional Council chief executive Fiona McTavish denied suggestions the council was in breach of its resource consent.
The consent required maintenance but did not stipulate a complete absence of corrosion or leakage.
She acknowledged accelerated corrosion was first noted in 2014, explaining the unique location – being the world’s only geothermal lake diversion wall – meant it was “not anticipated” microbial corrosion would shorten the wall’s lifespan.
“This sort of corrosion was not well understood or widely accounted for in infrastructure design when the wall was built,” said McTavish.
She cited monitoring by the University of Waikato that showed improvement in Lake Rotorua’s water quality, which meant the wall had less purpose.
She also suggested that council monitoring cast doubt on claims of leakage causing a rise in algal blooms.
“The cyanobacteria species in the Ōhau Channel and Lake Rotoiti are a different species. This information supports the earlier assessment that any leakage is not currently having an impact on water quality in Lake Rotoiti.”
The regional council continued to have a “management strategy” with the wall, including monitoring and risk assessment.
This resulted in repairs in 2019, and a polyethylene patching trial in 2023-24 that offered limited success. The council said the wall remained structurally sound.
“Given the current information, repairs are not urgent from either a water quality or a structural perspective.”
Trials of repair methods were under way, however, with costs ranging from $4.8m for minimal repairs to up to $62m for full replacement.
A total of $1.9m is budgeted in the long term plan for further trials in the next two years.
“We need to ensure any future options are fully investigated, and that the risks and costs have been assessed before acting,” said McTavish.
Once monitoring and design trials are completed, the regional council would discuss future funding requirements with Rotorua Lakes Council.
Lake Rotoiti Community Association deputy chairman Jim Stanton said a viable restoration solution, which would extend the wall’s lifespan to its original 50-year target, has already been provided. The estimated cost would be $25 million.
He wants the recommendation to be adopted at the next regional council meeting on August 7.
“This is not just a Rotorua issue, it’s a national environmental concern,” said Stanton.
“We need action now.”
Mathew Nash is a Local Democracy Reporting journalist based at the Rotorua Daily Post. He has previously written for SunLive, been a regular contributor to RNZ and was a football reporter in the UK for eight years.
- LDR is local body journalism co-funded by RNZ and NZ On Air.