Water engineers have been working through the night to restore supplies to 3,000 properties in the Carmarthenshire area.
Another 2,000 homes in the Powys area have also been affected by burst water mains, following record low temperatures and the thaw.
Dŵr Cymru Welsh Water said it had been dealing with 185 major bursts in a day around Wales.
Engineers are still working on supplies from Anglesey to Rhondda Cynon Taff.
Dŵr Cymru Welsh Water Operations Director Peter Perry said the company had been working at "full stretch" to rectify supply issues in Carmarthenshire.
"The teams we had on site have worked consecutively now for 36 hours," he told BBC Radio Wales.
"In the early hours of this morning the pumping station was brought back into commission and is now putting water back into our Foel service reservoir which supplies the bulk of our customers who were affected.
"At the extreme ends of the system people are not back in water, but they will be within a couple of hours."
Mr Perry said the issue at the Carmarthenshire pumping station serving areas around St Clears and the SA31 and SA33 postcode areas, had been a mechanical failure and not caused by cold weather.
"What made it difficult for us is that at this time of the year we are in that holiday period, so getting equipment to take the pumps out, getting them fixed and the ground conditions, which were very sodden in the area, caused a problem," he explained.
And, he added, since the Boxing Day thaw they are continuing to deal with a series of burst water mains across Wales.
"To put this in perspective, I've been in the business 30 years and I don't think we've ever had a winter like this," he said.
"That change in temperatures last week from minus 10 in our rural areas to plus 10 over the past couple of days has seen an unprecedented number of bursts."
Areas still affected stretch from the village of Llanddaniel on Anglesey in north Wales down to Aberdare in the Cynon Valley.
On Wednesday, around 2,000 homes in the Llandrindod Wells, LD1 postcode area of Powys faced disruption.
Dŵr Cymru Welsh Water said it logged between 5,000 and 6,000 calls from customers reporting problems since Boxing Day, compared with 600 on a typical busy day.
It is giving updates via its website to customers whose supplies are affected.