More than 100 workers have started on the second stage of the multimillion-dollar repair work on the Tekapo canal.
Owner Genesis Energy said the canal has had problems with leaks since it was opened in the 1970s but recent deterioration led to a $140 million repair programme being launched in 2012.
This week, repairs are being carried out on a stretch known as the Maryburn Fill and canal-lining experts from Europe have been brought in to carry out the work.
Mount Cook Alpine Salmon, which farms on the hydro canals, is having to move tens of thousands of fish to other sites.
Genesis Energy spokesperson Richard Gordon said Fish and Game staff will also arrive next week to help move wild trout and salmon to other parts of the canal.