People in the Guilsfield area were given bottled water to keep them going after the incident cut off supplies.
A spokesman for water company Hafren Dyfrdwy said repairs were taking place throughout the day following reports of a burst at 7.44am.
A Hafren Dyfrdwy spokesman said: “We would like to apologise to those affected by the burst pipe in Guilsfield early this morning.
“Our teams have worked hard to isolate the burst and repairs are now taking place."
The spokesman added: "The vast majority of customers affected are now back on supply, with bottled water being sent to the few that remain off whilst we carry out the repair.
“Again, we’re really sorry for any inconvenience caused and we’re working hard to get this resolved as quickly as possible.”