Lake Rotorua (Māori: Te Rotorua nui ā Kahumatamomoe) is the second largest lake in the North Island of New Zealand by surface area, covering 79.8 km2. It has a mean depth of just 10 metres and a considerably smaller volume of water than nearby Lake Tarawera. It sits within the Rotorua Caldera in the Bay of Plenty Region.
Geography Lake Rotorua is fed with water from a number of rivers and streams. Some, such as the Utuhina, carry water that is warmer than the lake, due to thermal activity in the Rotorua area. Conversely, streams on the northern shore, such as the Hamurana Spring and the Awahou stream, have crystal clear water with a constant temperature of 10 degrees Celsius. Other notable tributaries include the Ngongotahā stream, famous for trout fishing. Lake Rotorua flows directly into Lake Rotoiti via the Ohau Channel at the north-eastern corner of the lake. The urban development of Rotorua city extends along the south portion of the lake shore.
Geology The lake sits in the crater of a large volcano in the Taupō Volcanic Zone. The volcano's last major eruption was about 240,000 years ago. After the eruption, the magma chamber beneath it collapsed. The circular depression left behind is the Rotorua Caldera, which is the site of the lake.
The lake, as shown in Figure 1, has had periods at various surface levels since the caldera's Mamaku ignimbrite eruption, related to both volcanic activity and subsidence within the Taupō Rift. The lake's all-time high occurred sometime after the formation of the Ngongotahā Dome, which records this on its flanks, and is now dated at 200,000 years. Previously this dome's eruption was assigned much closer to the time of the Mamaku ignimbrite eruption, so the lake maximum high stand was assumed to be more directly related to the caldera's formation. Breakout floods are suspected through the Hemo Gorge after the lake's maximum high stand, definitely after the Rotoiti eruption of the Ōkataina Caldera, with formation of a channel within the western lake floor of Lake Rotoiti, and suspected after the Hauparu eruption through the Kaituna River Gorge. Several other lakes of volcanic origin are nearby to the east, around the base of the active volcano Mount Tarawera, and these have also varied in size or existence over time due to volcanic action. Mokoia Island, close to the centre of the lake, is a much later rhyolite dome than Ngongotahā. It is probably New Zealand's best-known lake island, and is closely associated with one of the best-known Māori legends, that of Hinemoa and Tutanekai. Is said that Hinemoa swam across the lake to her lover Tutanekai, who lived on Mokoia Island. Owing to the geothermal activity around the lake (including still active geysers and hot mud pools), the lake has a high sulphur content. This gives the lake's waters an unusual yellowish-green hue.
Mapping of lake floor In 2024, maps of the lake floor were published, showing the results of surveys carried out in 2016 and 2017. The maps reveal the course of an ancient river, now submerged, and thousands of pockmarks that probably result from the emission of gas from decomposition of organic material beneath the lake floor. The surveys also revealed significant geothermal activity extending around 1 km (0.62 mi) into the lake from Sulphur Point.
Pollution NIWA estimated that livestock farming around the lake adds over 650 tonnes of nitrogen a year, resulting in algal blooms and weed growth. Thus the lake's water quality is poor, with a Trophic level index between 4 and 5 (eutrophic). It takes about 60 years for inflowing nutrient-laden groundwater to reach the lake, so that restoration of the lake is likely to be slow, even if a target of 435 tonnes of annual nitrogen input is reached. Alum dosing of two tributaries reduces phosphorus levels and rules aim to reduce nitrogen inputs from the catchment and extend sewers to reduce septic tank inputs. In the 1960s it was noticed there had been a decline in kākahi and koura and an increase in invasive aquatic plants, mainly hornwort (Ceratophyllum demersum), Lagarosiphon major, Egeria densa and Elodea canadensis. Herbicides and mechanised weed cutters are used to control the weeds, sometimes resulting in complaints about smells. Annual weed removal reduces nitrogen levels in the lake. A storm in 2025 blew weed onto the lake shore near the town, requiring removal of about 725 tonnes.
Tourist attraction
Despite the large volume of water flowing through the lake, its shallow depth makes it very prone to discolouration, especially from sediment following windy weather. It is well used by fishermen, but less popular with watersports participants and swimmers. The Ohau Channel is navigable by boat and is also favoured by fly fishermen. The Ohau channel joins at the Mourea delta, an area with very low water levels. This area is frequented by novice kayakers and swimmers. Nearby is the ancestral land of the Ngāti Pikiao hapū of the Te Arawa tribe. From Lake Rotoiti the waters of Lake Rotorua flow to the Kaituna River and into the Pacific Ocean near Maketu; the rapid descent from over 900 feet above sea level in less than 20 km has created an area used for extreme kayaking and white water rafting.
Water transport
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