Swimming was not recommended in the popular Loch Laird, below the Benmore dam, at the weekend because of contamination.
Water tests last week by the Waitaki District Council showed bacteria levels (Escherichia coli) exceeded the limits recommended for safe swimming.
A notice advising people not to use the loch for swimming or water sports was published on Saturday.
The council's rural parks officer, Jane Matchett, said yesterday tests at the beginning of last week showed a high E. coli level, which was usual when water in the loch rose as a result of high water levels.
Last week, Meridian Energy released water down the Benmore dam spillway to ease the pressure on its Tekapo and Pukaki storage lakes.
Ms Matchett expected the water quality to improve.
