Roxburgh and Lake Roxburgh Village were put on boil-water notices on Wednesday, while Alexandra was added to the list on Thursday, following high water levels on the Clutha River.
Increased levels meant water supplies were at risk of containing harmful micro-organisms.
People in affected areas should bring tap water to a full rolling boil for one minute and then let it cool before using it.
A Central Otago District Council spokeswoman has also encouraged people to conserve water until the situation improved. Read more