People in Timaru are being asked to conserve water because the town has only enough reserves for four days.
Heavy rain has stirred up mud in the Pareora river, meaning the city has to rely on a backup reservoir.
Timaru District Council spokesperson Ashley Harper said if the weather kepts improving and everyone did their bit, they should not have to issue a boil water notice for the city.
"While there is no shortage of water in our two catchments, the level of silt in it means that we are unable to use it, as our treatment plant wouldn't be able to sterilise it completely," he said.
People are being asked to wait till the weekend to do their washing, turn off taps when brushing their teeth and take short showers.
Two thousand rural residents on the Timaru District's Downlands scheme are on a Boil Water notice, but all other rural schemes are operating normally.