Water supplies to 3,000 homes in Carmarthenshire have been affected by problems at a reservoir.
Dŵr Cymru Welsh Water said customers have been reporting loss of supply or low pressure caused by a burst main going to the reservoir.
The main areas affected include the postcodes SA31 and SA33 around St Clears.
Supplies to 2,000 properties in Llandrindod Wells (LD1) in Powys are also being hit by the weather.
Welsh Water is bring in waters in tankers there to help customers experiencing intermittent low pressure.
In Carmarthenshire, engineers have been working overnight to fix the problem with the supply to the reservoir.
“”I’ve been in the business for 30 years and the levels of bursts – we’ve never seen the like before”
Peter Perry Dŵr Cymru Welsh Water
Dŵr Cymru Welsh Water began giving out bottled water to customers on Wednesday morning.
Operations director Peter Perry said the company was working at “full stretch”.
“We are dealing with an unprecedented level of bursts not only on our system but also on the private supply side – on customers’ own pipes,” he said.
“Yesterday, we dealt with 250-plus bursts on our own system.
“I’ve been in the business for 30 years and the levels of bursts – we’ve never seen the like before,” he told BBC Radio Wales.
“Our biggest incident is in south Camarthenshire. There are about 3,000 properties without water. We are doing everything we can to get supplies restored.
“We’ve had a significant failure at a pumping station.”
There have also been reports of water problems in Rhiw, Gwynedd, and Miskin near Cardiff and Ynysybwl in Rhondda Cynon Taf.
Dŵr Cymru Welsh Water said it logged 5,000 calls from customers reporting problems on Boxing Day, compared with 50 on a typical busy day, and about 185 during a cold snap.
It is giving updates via its website to customers whose supplies are affected.
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