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A lake described as a “national treasure” faces environmental “disaster” if a lake wall is not fixed, advocates warn.
The Ōhau diversion wall, which stretches across 1.3 kilometres, aims to improve the water quality in Lake Rotoiti by stopping nutrients flowing in from Lake Rotorua.
But holes areappearing in the wall near the water’s surface.
After successive seasons of green water appearing in Lake Rotoiti, a group advocating to urgently fix the wall presented their case to the Bay of Plenty Regional Council on Wednesday as a submission to its draft Long-term Plan.
The $11 million diversion wall was built in 2008 and was expected to last 50 years but first showed signs of deterioration in 2014.
Don Atkinson, who owns a property at Lake Rotoiti, told Local Democracy Reporting after the hearing he had contacted the council to find out what was planned, when the work would be carried out, and how much money would be put towards it.
He said he was told $1.6m was planned for the 2025/26 year to place material over the holes that appeared to be allowing poorer-quality water from Lake Rotorua to flow through.
Atkinson, along with local retired structural engineer Geoff Dainty and Lakes Water Quality Society chairman John Gifford, urged elected members to bring the work forward a year and suggested $10m be put aside for a complete repair.
Lake Rotoiti is patchworked with algal bloom discolouring the water in parts. Photo / Laura Smith
Atkinson told the hearing they considered the planned fix a “sticking plaster”.
Lake Rotoiti was considered a “gem” but was “rapidly returning” to its prior state before the wall was built, when annual algal blooms prompted health warnings, property values dipped and the lake was in a degraded state, Atkinson said.
In his opinion: “It is an absolute disgrace we have infrastructure that has failed.”
Rotoiti was on the verge of lake-wide blooms, he said, and he believed it was “heading for disaster”.
Atkinson, Dainty and Gifford said in their view the council had failed by allowing it to happen.
Dainty said urgent temporary repairs were needed to allow time for the council to explore the root cause of the corrosion causing the holes.
Water at the Okere Inlet at Lake Rotoiti was the colour of the trees surrounding it in March 2023. Photo / Laura Smith
This would help determine the life expectancy of the king piles anchoring the wall to the lakebed.
It should then redesign and repair the wall, he said.
“The environment around there has caused the corrosion to be five times faster than what it was designed for.”
Gifford told councillors the society was “incredibly disappointed” repairs were not immediate within the draft plan.
Lakes Water Quality Society chairman John Gifford at Lake Rotorua. Photo / Laura Smith
He said it also recommended an urgent independent review on the wall’s “fitness for purpose” for the next 30 years.
The way the two lakes intersected was complex, he said, and while the impact of blooms in the warmer months was evident, the impact of bacteria during the cooler months was unclear.
“Until we have some evidence and information about the impact of nutrients, bacteria, algal assemblage and suspended solids moving into Lake Rotoiti there has to be urgency about making those repairs.”
Councillor Malcolm Campbell echoed their concerns.
“This is a national treasure. It’s going to turn back into … the shithole it was 10, 15 years ago.”
He did not believe the financial burden should be placed on those living around the lake, because it was widely used for recreational purposes.
Laura Smith is a Local Democracy Reporting journalist based at the Rotorua Daily Post. She previously reported general news for the Otago Daily Times and Southland Express, and has been a journalist for four years.
- LDR is local body journalism co-funded by RNZ and NZ On Air.
