Council water production manager Gerard McCombie said this meant watering the garden a little less, particularly in the area where supply was under the most pressure, Waikouaiti.
The call to conserve water came as recent hot dry weather affected demand and supply.
''The rivers and the creeks are well below normal in most cases and the water demands are up, particularly on the real hot days we have been getting over the last wee while,'' Mr McCombie said.
While the Mt Grand and Southern water treatment plants were still producing surplus water, the Waikouaiti plant was under pressure because of higher demand and the low level of the Waikouaiti River.
''That river is getting quite low and starting to head towards to the minimum flow, which would trigger mandatory water conservation measures like water restrictions,'' he said.
The situation would be eased if people dropped their water usage by 10% to 20%.
''Don't over-irrigate and do [water the garden] early in the morning or in the evening when evaporation is [less of an issue].''
Mr McCombie said he hoped some of the rain forecast for next week would eventuate.
''Hopefully, it's just a nice gradual, easy drizzle.
''We don't really want a big dump of it because it will just run off really quickly, dirty up the rivers and, from our perspective, it just makes the water a bit harder to treat.''
MetService's weather station at Dunedin International Airport recorded 31.2mm of rain in December, less than half the historical average of 66.9mm for the month.
In Dunedin city, MetService recorded 38.8mm, but it did not have historical records for that station.
