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On the job . . . Workmen construct a wooden penstock, used for transporting Oamaru's water supply, in 1917. A section of the penstock was recently uncovered during a work project in Chelmer St. PHOTO: NORTH OTAGO MUSEUM

An Oamaru historian believes the discovery of a long-forgotten, century-old section of vital infrastructure is a remarkable find.

Contractors who recently carried out stormwater pipe work in Chelmer St uncovered a historic penstock used to carry water that was part of the town’s drinking supply.

A penstock is a wooden pipeline used to transport water, a system still used today in some places.

The section uncovered in Chelmer St, about 1.5m underground, was constructed in 1917 and is about 90cm wide.

Local historian Bruce Comfort said hundreds of metres of timber planks were used to construct the pipe, which was held in a circular shape by thousands of iron hoops that were secured to it.

He said the find was exciting.

“Wooden pipelines were, and still are, state-of-the-art for transporting water, but the penstock was buried after testing and that is rather unusual.

“It was and remains unique in the world for its time, inasmuch as the water supply for the power station was the public drinking water supply, and the water was sent to waste after use.”

He said the pipe was a significant part of the history of the town’s public water supply, which dated back to the mid-1870s.

In 1874, the Oamaru Municipal Council commissioned its engineer, Donald McLeod, to design a public water supply for Oamaru for both domestic and industrial use, and firefighting.

The water supply was made up of an open water race from the Waitaki River near Duntroon, made up of about 49km of open channel, timber aqueducts and five tunnels, one of which was close to a kilometre long.

When Oamaru’s water supply was established, spare water was used to power the town.

Small turbines were installed around Oamaru to produce DC electricity for lighting.

In 1881, due to increasing demand, a second, cast iron supply pipe was laid from the Ardgowan reservoir, which is still in use.

By 1913, technology for producing AC electricity had developed enough for the Oamaru Borough Council to contemplate generating its own electricity and so limit the direct use of its own town water supply, diverting the water to electricity – a better source of revenue.

However, the outbreak of World War 1 meant plans for a power station were slowed and a purpose-built generator and turbine was not delivered from Britain.

Instead, a second-hand unit was brought together and started to generate power at a Chelmer St building, now the site of Milligans Food Group, in 1917.

The water to drive the wheel was brought over Hospital Hill through the penstock.

The penstock and power plant were decommissioned in 1954.

“The power station was not among New Zealand’s very earliest electric power generating undertakings,” Mr Comfort said.

“However, it was probably the fourth [in New Zealand] undertaking to supply AC-distributed electricity constructed with public money by a local authority.”

After the section of the penstock was exposed last month, he recovered some of the timber and 10 of the iron hoops, as well as parts used to secure them to the penstock.

He donated one of the hoops to the North Otago Museum.

Components . . . Penstock timber, a section of an iron hoop and a part used to secure it that were recovered by Oamaru historian Bruce Comfort. PHOTO: SUPPLIED