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, causing it 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.
The problem was first discovered in July when it appeared the dam was losing up to 30 litres of water per second.
The council was unable to estimate how much water was lost as the amounts fluctuated.
It also took time to determine the source of the leak.
"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.
Council could not say with full confidence the precise sequence of events leading to the major leak.
However, various possibilities were being assessed and mitigation strategies prepared to prevent a repeat situation.
Council said the impounded supply was now being filled and would be monitored closely for any further leakage.
Improvements to monitoring devices on the cover and at the drain discharge were being completed.
"We would like to thank everyone for their understanding and patience during this unprecedented event.
"We are still exploring ways to reduce pressure on our water supply without increasing restrictions for urban users.
"We are unsure what this looks like yet but will provide you with an update when we can."