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The deactivated cast iron oval spent many years bobbing on the Waikato River as a buoy, but has now found a new home at the museum in recognition of Akaroa Harbour’s defensive minefield history.
Museum director Lynda Wallace said the mine is the same as those laid across the harbour in January 1943.
"Akaroa was the only South Island harbour with mines,” Wallace said.
"At the height of World War 2, it was protected by a row of 64 mines, each containing 500lb of TNT.
"Known as J.L.8, the minefield was laid out in four loops of 16. Along with nearby infrastructure, it formed part of New Zealand’s strategic coastal defences during the war.”
The Navy could detonate the controlled mines from the shore base following any sighting of an enemy vessel – including a submarine – creeping into the harbour.
Costing £18,240 and completed in 1943, the Akaroa Battery included roads, a jetty, gun emplacements, searchlights and an accommodation block, along with the minefield.
"Today you can still see the old depot – a large building across the harbour from Akaroa in Tikao Bay – built to store the sea mines,” Wallace said.
However, the Akaroa minefield soon became a “white elephant”.
Just over a year after the mine-layer, Alsey, and the base ship, Atreus, put down the field, the Defence Department downgraded the threat of an invasion by sea and ordered the Navy to blow up the mines.
"Some older Akaroa residents can still recall the huge explosions that echoed around the harbour on 21 February, 1944,” Wallace says.
"All but two of the mines were detonated.
"However, the Navy concluded that those mines had probably flooded and would not pose a risk to shipping.
"Just a few years later, a fishing boat working off the heads of Akaroa Harbour reported a floating mine. It was soon sunk by the Navy offshore.”
Wallace said the sea mine is being stored behind the scenes in readiness for a new museum exhibition which aims to re-tell the story for new generations.