Western Southland's Monowai Hydro Power Station, one of the oldest in the country, is turning 100 and owners Pioneer Energy plan to mark the milestone by holding a special Open Day on Thursday, May 1, 2025.

The outlet of Lake Monowai into the upper Monowai River, which was modified in 1922 by a low earth embankment and a series of control gates to feed water into the Monowai Power Station. Photo: Pioneer Energy/Supplied

Fiordland's Lake Monowai was first considered as a site for a future hydro scheme as early as 1910, due to the natural rate of fall of its outlet river, with serious investigations finally commencing in 1919.

Major construction was eventually started by the Southland Electric Power Board in 1922 and included the establishment of a new Monowai Village for worker accommodation.

Materials and workers were transported from the closest railhead, at Tuatapere, before being moved to the construction site by horse-pulled carts, trucks and traction engines.

A low earth embankment dam and control gates was formed to control the lake's level.

Water from the lake would be channeled 6km down the Monowai River to a small lake before entering a 865m canal to a reservoir (forebay).

From there 2.6m pipes would take the water 1036m to a surge tank before eventually entering three 1.6m pipes (penstocks) to feed the power station's turbines.

Construction of the Monowai Hydro Power Scheme in the 1920s. Photo: Pioneer Energy/Supplied

Once the water had turned the turbines it would be discharged into a tail water pond before flowing over a weir and back into the Waiau River.

By December 1924 the project was complete and featured two machines. It was officially opened on 1st May 1925.

However in 1926, with demand beginning to exceed supply, engineers controversially decided to raise the level of Lake Monowai by 2.13m (7 feet) in order to improve the stations overall power generation capacity as well as ensure a more reliable water supply in dry periods.

As a consequence, vast areas of Lake Monowai's pristine foreshore were flooded, causing outcries from both the community and environmentalists and leaving an ecological disaster that is still evident today.

In 1927 a third machine was commissioned.

Electricity from the station was used exclusively by Southland homes and farms but after the government's purchase of the plant in 1936, it was subsequently linked up to the national grid (1939).

Monowai Power Station looks the same as it did 100 years ago, but upgrades mean it can now run automatically and is not attended. Photo: Pioneer Energy

In 2002 the present owners, Pioneer Energy, purchased the scheme.

Pioneer Energy have since carried out extensive upgrades to the scheme since, with new turbines, generators and control systems all installed. A fish pass now also operates within the scheme.

The station currently generates 36 gigawatt hours per year, and while still operating from its original 1925 building, it is now designed to run automatically in an unattended mode.

Monowai Hydro Station 100-year Celebrations

Date: Thursday, 1st May

Programme: 10am, Speech & Power Station tour; 12pm, Refreshments at Brunel Peaks Cafe

RSVP: [email protected]