Water bowsers were deployed last week in Carmarthenshire About 200 homes in Powys are still without water as engineers continue to try to restore supplies.
Welsh Water had hoped to restore supplies to the properties in St Harmon and Abbeycwmhir, near Rhayader, on Saturday.
It now aims to have fixed the problem by Sunday evening.
A spokesman said “99.99%” of customers now had water and its emergency response level was the same as it would be on a “normal working day”.
Welsh Water has apologised to anyone affected.
Peter Perry, Welsh Water’s operations director, said fewer than 40 properties in south Carmarthenshire may be experiencing low pressure or intermittent supply due to its systems returning to normal operating pressure.
Bottled water is being distributed to those affected.
Mr Perry said there were also “isolated incidents” affecting single properties or small numbers of customers across the region.
He said fixing bursts throughout the system and restoring reservoir levels back to normal was now a priority.
He added: “I would again thank customers who have suffered a loss of service or a poor service for bearing with us during this very challenging period.”
He said work would continue over the Bank Holiday period to try to restore supplies.
He added: “Finally I would like to thank all our own staff who have shown fantastic commitment and resolve during what has been said is the coldest December on record.”
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