Final Report – Experts Ohau Wall Findings

The final full Report is HERE.

Key Findings

Lake Rotorua has much poorer water quality and more nutrients than Lake Rotoiti, so water flowing from Rotorua into Rotoiti caused serious water-quality problems in the past, including increasingly frequent toxic algae (cyanobacteria) blooms.

To stop this, the Ōhau Channel Diversion Wall was built in 2008. It was designed to send most of the Rotorua water straight down the Kaituna River rather than allowing it to enter Lake Rotoiti.

The wall worked well initially. After it was built, the amount of Rotorua water entering Rotoiti fell substantially, and Rotoiti's water quality improved.

However, the diversion wall is now deteriorating. The steel sheet piles have developed an increasing number of holes, allowing more Rotorua water to leak through into Lake Rotoiti.

The study used long-term water-quality data and a computer model to estimate how much Rotorua water is now getting through the wall. It found that:

• • Before the wall was built: approximately 50–80% of the water entering this part of the system could be attributed to Rotorua water.
  • After the wall was built: the intrusion was greatly reduced.
  • More recently: the estimated intrusion has increased to approximately 20–35%.
  • The amount varies with the seasons and weather. Heavy rainfall and high flows increase the amount of Rotorua water getting through the wall.
  • The problem is therefore not constant — it can become significantly worse during high-flow events.

The key warning

The modelling suggests that if the current deterioration continues, the amount of Rotorua water entering Lake Rotoiti could return to something approaching the pre-2008 level in about 10 years.

In simple terms:

The diversion wall was successful in protecting Lake Rotoiti, but it is now failing. An increasing amount of nutrient-rich Lake Rotorua water is getting through, and if nothing is done, the protection provided by the wall could largely disappear within about a decade.

Why this matters

The concern isn't simply that more water is leaking through the wall. Rotorua water carries higher nutrient levels, which can encourage algae growth and reduce water quality in Lake Rotoiti.

The study therefore concludes that the condition and future of the diversion wall need to be reviewed urgently, because the longer the deterioration continues, the greater the risk of Lake Rotoiti returning to the poor water-quality conditions experienced before 2008.

Final Complete Report

The full final report is attached including tables - Professors report - Water Intrusion through Oahu Wall into Rotoiti - Australian Rivers Institute - Jun 2026