Up to 40,000 homes are without water in two counties and two schools have been forced to close due to a burst main at a treatment plant
The incident at the Bryn Cowlyd Water Treatment Works in Dolgarrog, Conwy, is affecting people in Conwy county as well as Denbigh and surrounding areas, Welsh Water said.
It added that tankers were being mobilised to keep as many people on supply as possible.
The company said its crews were working on repairs which were expected to be completed by 09:00 GMT.
The incident is affecting supplies in Conwy, Dolgarrog, Eglwys Bach, Groesffordd, Gwytherin, Henryd, Llanbedr y Cennin, Llanddoged, Llanfair Talhaiarn, Llangernyw, Maenan, parts of Llanrwst, Pandy Tudur, Pentrefelin, Rowen, Rhyd y foel, Tal y Bont, Tal y Cafn, Tyn Groes and Trofarth.
Welsh Water said customers had been contacted directly by text message to inform them about the issues.
