Lake Rotoiti is located at the northern end of Lake Rotorua and these two lakes are connected by the Ohau Channel which means that the quality of the water in Lake Rotorua has a significant effect on Lake Rotoiti. Lake Rotoiti is a relatively large lake and a popular location for boating and fishing. Much of the work being done to improve Lake Rotorua is also aimed at improving water quality in Lake Rotoiti. The lake has a total size of 3400 Ha and the catchment is 12160 Ha. The average depth of the lake is 31 m and at its deepest point it is 93.5 m. The lake drains into the Kaituna River which flows into the sea at Maketu.
The full name of the lake is Te Rotoiti-kite-a-Īhenga, which in Maori means "The Small Lake Discovered By Īhenga", the explorer also credited with discovering Lake Rotorua. Legend says that the lake was named as such because when Ihenga first saw it, he was only able to see a small part of it and thought the lake was a lot smaller.
In July 2008 a diversion wall was constructed that prevents water that has a high nutrient content from flowing directly into Lake Rotoiti. As a result more than 70 percent of the nutrients that were flowing into Lake Rotoiti have now been diverted down the Kaituna River. The resource consent for the Okere Gates also requires these to be operated to ensure water quality in Lake Rotoiti, by preventing backflow into this lake.
The Ohau Diversion Wall is diverting Lake Rotorua's nutrient rich waters away from Lake Rotoiti. This intervention is protecting water quality in Lake Rotoiti while actions are implemented to improve Lake Rotorua's water quality. The long term plan for Lake Rotoiti is to eventually remove the Ohau Diversion wall, once Lake Rotorua water quality has sustainably improved, enabling the waters to mix naturally again.The 2015 TLI for Lake Rotoiti was well above the target TLI due to a long hot summer. However, lake water quality needs to be considered by a rolling average rather than annually to get a true picture of water quality conditions. This is because annual readings can be influenced by one off climatic conditions. The 3 year rolling average TLI for Lake Rotoiti is around its target of 3.5 and the lake water quality is considered stable.