Up to 40,000 homes in two counties could be without water, and five schools have been forced to close after a main burst at a treatment plant.
The incident at the Bryn Cold Water Treatment Works in Dolgrog, Conwy is affecting people in County Conwy as well as Denbigh and surrounding areas.
About 8,000 households lost supplies on Wednesday night and about 33,000 more properties were at risk of losing supply until repairs were completed, the firm said.
I five schools Conway will remain closed because they have no water and the problem is not expected to be resolved until after Thursday.
Welsh Water said tankers were being mobilized to keep as many people on supply as possible.
The company added that its crews were working on repairs, but that the work was “difficult and dangerous” because the burst water main was two and a half meters below the river bed.
The company added that crews would have to build a dam-like structure in the river so they could excavate and gain access to the damaged water main.
The incident affected supplies in Conwy, Dolgrog, Igloisbach, Grosford, Guthrin, Henreid, Llanbedr and Senan, Llandudgade, Llanfair Talhairn, Llangarnew, Mennan, parts of Llanrost, Pandy Tudor, Pendy Tudor, Pantry, Pendy, Rutney, happening Bonnet, Tall y Caffin, Tyne Groves and Trowforth.
Welsh Water said customers were contacted directly via text message to inform them of the problems and that bottled water was being delivered to vulnerable customers.