World War II naval sea mines heading from Waikato River to museums

Mercury resource manager Alan Hurcomb (left) and hydro generation manager Andrew Peckham with three of the former World War 2 naval sea mines being donated to museums. Photo / Laurilee McMichael

Those things holding up the booms that run across the Waikato River in front of the Taupō Control Gates bridge which stop boats going over the control gates.

They look like big plastic floats. They float like big plastic floats. But they're not big plastic floats.

They're actually steel World War II naval sea mines.

Without the explosives, obviously.

Wikipedia says "a naval mine is a self-contained explosive device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, any vessel."

So what on earth are old sea mines doing floating in the Waikato River? Good question.

Nobody really knows, but Mercury hydro resource manager Alan Hurcomb has found out a little bit of information about their provenance. He say the mines were originally deployed in Wellington Harbour during World War II as part of defences and then disarmed once the war was over. In other areas, such as Akaroa, the mines protecting the harbours were blown up at the war's end.

Following the war, hydro dams were being built on Waikato River for electricity generation and construction materials were in short supply. But typical Kiwi ingenuity meant anything handy would be utilised.

"We suspect they were brought up during the construction of the post-war dams," says Alan.

"My guess is that post World War II materials were extremely hard to find when they were building power stations and, 'here's some mines, we'll use these'."

In all, Mercury has around 28 of the former sea mines, with around 15 of them still in use on the two booms at the Taupō control gates and 13 more in storage. A couple will be retained as spare floats but as part of a clear-out, Mercury is donating the historic artefacts to interested museums around New Zealand, with mines so far heading to museums in Wellington, Gisborne and Akaroa.

"They've laid around for so many years we've decided they're a nuisance and they're surplus to our requirements," explains Alan. "We do need to keep a couple of spares for the Taupō boom although there are great materials that have now superseded these."

The 200kg sea mines have been regularly refurbished and repainted over the years and small circular welds near the top of each mine mark where the Hertz horns, the protuberances that would trigger detonation when a ship bumped into them, used to be.

Alan, who has been with Mercury for more than 30 years, says he and other staff have always been aware that the floats on the booms at Taupō were former sea mines, lashed together with wire rope and attached to anchor points on the river banks. The booms mark a 200m exclusion zone that boats, swimmers and other river users are not permitted to enter due to the dangerous currents created by the hydro infrastructure.

Alan says the Taupō floats are in good condition and the floats, ropes and anchor points are inspected and maintained regularly so will stay in place for the foreseeable future. They also do a good job, he added.

"We've had boats that have broken their moorings and have been hooked up and stopped by the booms...they're pretty good."

One of the former naval sea mines which was repurposed as a river float for more than 70 years and is now being donated to a museum. Photo / Laurilee McMichael

Mercury hydro generation manager Andrew Peckham says once the explosives were removed from the sea mines they made excellent floats, and despite being more than 75 years old, they are made from good-quality steel that has lasted well.

"They are pretty robust so we keep on using them because they stand up to the job."

He says the reuse of the sea mines as hydro booms is a great example of Kiwi resourcefulness and it's wonderful to be able to send them to museums so that the stories can be shared.

"It's great that these ones are being preserved."