Ice and debris have reduced the inflow of water to "critically low" levels at the intake of the Moa Flat rural water scheme.
A spokesman for the Clutha District Council said consumers on the scheme should conserve water immediately.
The conserve water notice affected households and farms from the Moa Flat, Edievale, Heriot and Crookston areas.
Water level was critically low, with only 38 days’ supply left in the raw water reservoir.
Ice and debris were being blamed for reducing water flow at the intake.
It was estimated 691 cubic metres of water entered the reservoir each day but 1800 cubic metres was being used.
Urgent reductions in consumption were needed to avoid running out.
The conserve water notice will be lifted once ground conditions allow safe access to the site and normal water flow can be restored.
Residents were advised uses such as watering gardens or cleaning cars were prohibited until further notice.
Farmers were advised to check for leaks and turn off any systems not deemed critical.
