The Dannevirke impound water dam during its construction phase. Photo / NZME
The one in 50-plus year drought last year is thought to be the reason behind leaks at Dannevirke's impounded supply.
A Tararua District Council spokesperson said the drought caused the ground to dry and contract.
Paired with the lowering reservoir level, that exposed the plastic liner on the reservoir, causingit to vibrate and wear until it failed from fatigue.
Splits found in the plastic liner allowed for water to start removing the protective clay layer below, which then exposed the liner to underlying stones which increased the damage.
The leak became obvious when the reservoir began filling, the spokesperson said.
"With over 22,000 square metres of surface area, there has been a lot of area to assess," council group manager of infrastructure Chris Chapman said.
"Initially it was like looking for a needle in a haystack."
Council called in experts to determine the source of the leak.
An underwater drone was also used to assess the liner in the reservoir.
A second drone identified a tear in the cover and some small holes.
This area is still awaiting repair due to weather conditions.
The council spokesperson said up to four tonnes of protective lime had been replaced and compacted below the liner covering the hillside metal that was exposed.
An extra thick layer of geomembrane material had been installed to mitigate the problem from reoccurring.