Tararua District Council is developing a contingency plan following four recommendations from experts investigating ongoing leaks at the Dannevirke dam.
The council had been monitoring seepage from the water supply reservoir following repair of a leak in 2021.
The initial leak was discovered in July last year and experts were brought in to locate it and investigate the cause before repairs could be done.
However, despite those repairs, monitoring revealed irregular and abnormal seepage.
The council engaged environmental and engineering consultants Tonkin & Taylor, drawing on their expertise in water storage dams.
A council spokesman said the consultants completed an initial site inspection this month and were undertaking further reviews and analysis.
They also made preliminary recommendations.
- Operate the reservoir at lower water levels
- Undertake targeted monitoring
- Develop a contingency plan
- Prepare for repairs.
Council chief executive Bryan Nicholson said he wanted to reassure Dannevirke residents that their water supply currently remained stable and was safe to drink.
"However, we do want to be transparent and make our community aware of our challenges and any associated risks," he said.
"With dam safety, it is necessary to err on the side of caution when there is uncertainty."
The consultants recommended operating the reservoir at lower water levels, which would reduce the leakage risk while the investigation was ongoing.
The reservoir was around one-third full and council staff were reviewing historic trends to confirm the optimal water level to balance risks for secure water supply.
The spokesman said the council had stepped up monitoring to daily instead of weekly, which was "quite comprehensive."
Monitoring was intended to provide early detection of onsite changes that might indicate an escalation of the leakage risk to a more serious situation.
Council was also developing a contingency plan in case the situation deteriorated.
Development of the plan involved defining the situations and conditions under which to activate the plan, identifying responsibilities and actions to mitigate risk if the situation worsened and having systems, equipment and plant at the ready.
Repairs would likely require an extensive programme of works as well as improvements to the current system to minimise disruption to water supply.
The council spokesman said it was a matter of timing.
"When they've decided what the best course of action was, what plan they're going to follow to remedy this issue, the next question will be when do we do this.
"Are we going to empty the dam [in] summer or are we going to wait until the autumn?"
He said the council was looking into what it could do to maintain supply, such as bores or building more treated water storage tanks.
"There's a whole host of things they're looking at as considerations, but nothing concrete yet."