A giant 104-tonne electrical transformer, imported from Australia and transported across Southland, is set to help regain some of Meridian Manapōuri Power Station's lost capacity.

The 55-year-old Manapōuri Power Station, the largest hydroelectric station in New Zealand, has been operating at reduced capacity for around two years after faults were discovered in two of its seven transformers.

The 135 MVA transformer was unloaded from a ship in Bluff before making the 170km journey overland to Supply Bay in Manapōuri.

Meridian staff delicately reverse the 104-tonne transformer down 2km of tunnel to the Manapōuri power station's machine hall. Photo: Supplied

From there it was transferred to a barge - the first time Meridian has moved a load of this weight by barge - before completing its journey across Lake Manapōuri to the station.

A Meridian spokesperson said the aquation of a higher load capacity barge meant Lake Manapōuri, instead of Doubtful Sound and Wilmont Pass, could now be used for heavy loads.

Once docked at West Arm, the transformer was delicately reversed down a two-kilometre road access tunnel into the underground power station's cavernous machine hall.

Meridian acting GM Generation Yanosh Irani said the operation had required plenty of early starts and late finishes.

“This has been months of work, planning, and coordination, and the specialist skills on display have been amazing - Meisters for barge operations, Move Logistics for transportation from Bluff to Supply Bay and then again at West Arm, and E-Type for fabricating and installing two unloading wedges for the delivery."

"A real – and world class – team effort,” he said.

“We saw this winter just how important every single megawatt is, so this transformer has landed at a brilliant time.”

The new transformer is maneuvered into position in the power station machine room. Photo: Supplied

“Getting our biggest power station closer to full capacity will boost security of electricity supply next winter and, in the meantime, give us the ability to ease off generation in the Waitaki to help restore storage levels in Lake Pūkaki,” Irani said.

The new transformer will increase generation capacity at Manapōuri from the current restricted limit of 640 MW to around 768 MW – close to the maximum 800 MW allowed under its consent conditions.

Work to commission it is expected to be completed by Christmas, and the second replacement transformer (along with a spare) is due to arrive in late 2025.