Dunedin residents may have to be mindful of their water usage for up to a year.
Seventy-five percent of the city's main catchment has been burned out in a massive blaze at Deep Stream near Middlemarch.
Dunedin Council's '3 Waters' group manager Tom Dyer says backups are in place, but capacity is still compromised.
"They are viable long-term options, but it just means we're down to one less level of redundancy than we would normally have."
Dyer says officials will have a better idea of a timeframe by the end of the week. For now, residents need to be mindful.
"We'd be asking the residents of Dunedin at this stage to voluntarily manage their water consumption and not use anything to excess - that just means not excessively watering gardens or lawns, or anything like that."
A large cloud of smoke was visible over much of south Otago on Saturday as the blaze burned thousands of hectares.
Dyer says there's not a lot they can do.
"About 75 percent of the catchment has been burnt out, which means a lot of it will just be a waiting game for the vegetation to grow back, and all of the soot and sediment in the catchment to settle."
