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The last phase of a $41 million refurbishment of the Waitaki Dam and powerhouse is due to start soon, with the overall project taking longer than owner Meridian Energy expected.
It will be the dam's biggest upgrade since it was commissioned about 81 years ago.
More than two years has already been spent on the project. It was initially expected to be completed towards the end of next year.
However, due to the complexity of the works within the dam and the limited space available, the project was now due for completion at the end of 2017, six to 12 months later than originally planned, a Meridian spokeswoman said.
The project is part of a programme by Meridian to upgrade its generation assets.
Aviemore, Manapouri and Benmore have already been done and, after Waitaki, the company will look at Ohau A, B and C on the upper Waitaki scheme.
The final phase of the Waitaki dam project will be structural safety work, identified as part of a dam safety assurance programme evaluation.
The evaluation found the dam was structurally sound and met current safety criteria for normal and extreme loads.
However, in a significant seismic event - a one-in-2500-year earthquake - there was potential for movement between the 21 blocks in the 350m-long, 36.5m-high dam which could lead to flooding of the lowest galleries.
Measures would be taken to mitigate and control any problems, including installation of bulkheads and a piped manifold system, excavation of sumps, new pumps and delivery systems and new lining and seals.
The work would have to be done in narrow galleries about 1.2m wide and 2m high.
The galleries provided access within the dam structure and manage any pressure from under its foundation.
Meridian has advertised for interested parties for the detailed design and construction of the structural safety works.
The full cost of that part of the project is estimated at several million dollars.