Work being carried out on the country's largest irrigation scheme is aimed at making it more efficient.
A 1km section of the Rangitata Diversion Race has been lined to prevent leaking.
A geo-membrane has been laid as a base for another liner with a bentonite clay centre. Bentonite clay expands when wet to form a seal.
This has been covered with sand and compacted topsoil about 300mm deep.
It is hoped this will eliminate the loss of 500 litres of water a second, operations manager Neill Stevens said.
Because a leak was in a section of the race it could not be pinpointed, a kilometre length was lined.
The race had been closed since August 9 but was on track to reopen on Monday,Mr Stevens said. It was generally closed down once every three years for maintenance.
In a section near to the leak, a clay base has been added to help with sealing and the concrete floor of the intake has been resurfaced after it was worn away by gravel movement.
Divers cleared debris from the intake to ensure the gates formed a good seal when closed.
Parts of the race where there was seepage were under surveillance, Mr Stevens said, and the ongoing process of eliminating leaks would target these areas.
''The race is becoming more efficient,'' he said.
The race travels 67km, from the Rangitata River to the Rakaia River, feeding three irrigation schemes, two power stations, the Ashburton District Council stock-water races and some private water races and irrigation schemes.
It can draw 30.7 cumecs of water from the Rangitata River and a further 7 cumecs from the Ashburton River but must not draw more than 35.4 cumecs in total.