Ōamaru's water supply may run out within two days, unless water restrictions are observed.
The Waitaki District Council said the town's reservoir was dangerously low with far less water being produced than required.
Full water restrictions are in place for Ōamaru and surrounding areas on the town supply, and people are being told to use water for essential use only.
If the town runs out of treated water, the council will be forced to deliver untreated turbid water, that will have to be boiled to drink.
That would likely mean schools and businesses would have to close.
Waitaki mayor Gary Kircher said recent heavy rainfall meant the water going into the treatment plant was dirtier than usual.
"We've just got a situation at the moment where we've got water entering out retreatment reservoirs about three times normal turbidity, and even our plant is struggling to keep up with that.
"From that point of view, it's pretty significant, and we're just not keep up the demand for treated water."
A council spokesperson said water usage needed to be seriously reduced for four days to let things recover to a manageable level.