Electricity generation in the Wairoa River catchment had its beginnings in 1915 with the construction of a 750 kW plant at Omanawa Falls, followed in 1925 by the commissioning of the McLaren Falls Station.

Today, the scheme consists of the 0.3 MW Kaimai 5 Station on a diversion tunnel feeding Lake Mangaonui, the 16 MW Lloyd Mandeno Station, sited in the west bank of the Mangapapa River, the 5.6 MW Lower Mangapapa Station, and 4 km’s further downstream, the 20 MW Ruahihi Station. The total annual output of the scheme is 167.8 GWh.

The McLaren Falls Station was decommissioned in 1989 following commissioning of the Ruahihi Station, with a bypass installed to allow the release of recreational flows into the Wairoa River on set days each year.  

The development of the scheme has resulted in an easily accessed recreational area, used for fishing, kayaking, picnics, and camping. Water is released through the McLaren Falls bypass up to 26 days each year; this provides high flows for river rafters and the Kaimai Canoe Club.

The occasional spraying of pest plants in the Ruahihi Canal and Lake McLaren is carefully managed to ensure that any potential adverse environmental effects associated with this action are avoided. These control measures have direct benefits for the ongoing use of the conveyance and storage facilities for both energy production and public recreation.

The Resource Consents for this facility will begin to expire in August 2026.

Lloyd Mandeno is the most up stream station. A lake has been formed by building a 29m earth dam across the Mangaonui valley, this creates the main ponding reservoir for the station. The Lake has a surface area of 8.1ha and an operating range of 3m. There are 9 km of diversion tunnels in this part of the scheme. Water flows from the lake into the canal through a feed tunnel that is regulated to maintain a constant water level in the canal and constant head on the machines. In the power station there are two sets of turbines and generators, each producing 7,800kW at 11,000 volts. Operating speed is 750rpm and each set uses 6.1 cubic metres per second at full load.

The Lower Mangapapa Power Station is situated on the right bank of the Mangapapa River 4km further downstream from Lloyd Mandeno. The station was constructed after the summer of 1976 and was completed in the April of 1979.

This section has a concrete arch dam built across a narrow gorge in the Mangapapa River. The dam is 26m high forming a lake that extends upstream to the Lloyd Mandeno station.

Water is taken from the lake through a 3m diameter concrete lined tunnel 400 m in length. A steel tunnel outlet into the station divides to supply two horizontal turbines which drive a centrally located 6,000 kW induction generator. The combined output of the turbines is 6250kW at 32.8m head. The unit has an operating speed of 375rpm and a water consumption of 21.5 cubic metres per second. The average energy produced is 16.7 GWh per year.  

The Ruahihi Power Station is situated on the Wairoa River adjacent to SH29. Ruahihi is the third and largest section of the overall scheme. Construction contracts were let in mid 1977, and the station was commissioned in 1981, but a failure in the feed canal later that year required major rebuilding. The station was recommissioned in 1983.

The reservoir for this station is Lake McLaren and the canal links the reservoir to the station. Lake McLaren was formed in 1925 by the construction of a 26m high concrete arch dam across the lower Mangapapa River to operate the now decommissioned McLarens Falls Power Station. Water from the lake passes through a gated inlet structure into a 2.5 kilometre long canal. The construction of the canal involved moving 2, 400, 000 cubic metres of soil at depths up to 46 m below original ground level, making it one of the larger canals in New Zealand. The depth of the water in this canal is 6m and the width at normal operating level is 30m. Flow velocities are up to 0.9m per second depending on machine settings and water levels.

Transition from the canal to penstock is a forebay which again has screens, a cleaner and control gates. Downstream of the forebay is a 1.6km low pressure conduit leading to twin high pressure penstock pipes down the escarpment and under State Highway 29 into the power house.

There are two generating sets in the station, each producing 10, 000kW at 86.4m head of water. Operating speed is 500 rpm and the average energy produced is 75.6GWh per annum.