In a letter to residents, Welsh Water chief executive Peter Perry said the weather gauge at Tynywaun had measured “the second highest rainfall in the UK last Saturday and Sunday”.
“This resulted in a significant amount of flooding (at the treatment works) and surface water running from the hill onto the drinking water storage tank and impacting the tank.”
Mr Perry said the required fix to reseal the tank would usually take “over a month”, but staff were working “flat out” to try to get it done in two weeks.
Ongoing rainfall had hampered efforts, as dry conditions were needed to lay protective membranes, he added.
“We understand the inconvenience this has caused for customers… and we’ll do all we can to bring that boil notice off as quickly as possible.”
A further £100 is now being offered to affected households, on top of £150 already pledged, while businesses will be offered a total of £500.
Meanwhile, Rhondda Cynon Taf council is offering a £1,000 grant for affected businesses and residents, and it said on Thursday that 192 applications had been received so far.
The council said that about 249 properties were hit by flooding inside their premises due to Storm Bert.
A further 140 resident and business properties have been flooded across Caerphilly, according to the council.
Elsewhere, Powys council have confirmed that 16 properties have suffered flood damage. Powys does not currently have a financial assistance scheme, but is seeking support from the Welsh government, and anyone hit by flooding can apply for council tax relief.
