Clutha District Council announced earlier this month it would drain the 160-year-old Phoenix Dam in Lawrence by an initial 3m to address concerns raised during geotechnical investigations.
Council service delivery group manager Jules Witt said the operation was being undertaken to "significantly reduce the impact of risk of any potential failure of the dam".
It was originally constructed for gold mine sluicing in 1863.
It had been assessed as higher risk under the Building Act, subsequent to investigations by Riley Consultants in 2021, he said.
"Riley was initially commissioned by [the council] and then later by [site owners] Port Blakely Limited to carry out preliminary geotechnical investigations, and perform preliminary stability and spillway capacity analyses to determine whether the dam meets the criteria for being ‘dangerous’, ‘flood-prone’ or ‘earthquake-prone’ under the Building Act."
As a result of those investigations, stakeholders had begun a controlled emptying/dewatering operation to initially lower the level of the dam by 3m.
"[We are] working with the owner of the Phoenix Dam, the Otago Regional Council as the regulator, and Emergency Management Otago to ensure the dam’s continued safety.
"Work ... involves water being drained by siphon from the dam to the creek below ... The main equipment for the operation was transported to the dam by helicopter and is being installed, along with flow meters to monitor the water flow."
Although the operation was likely to take several months to complete, residents should not be concerned.
"The dam site is not earthquake prone, however there is a flooding risk at the area. There is no evidence of overtopping of the dam recorded since 1864."
No impact on water supply for Lawrence was expected, but the council would closely monitor the situation as emptying continued.
Residents should avoid the area during works.