The Water Service in Northern Ireland has caused more than 1,500 pollution incidents in the five years to 2005 - almost one a day.
The service was blamed for a major fish kill
The figures were revealed under the Freedom of Information Act following a request from BBC Radio Foyle.
The incidents of pollution include a major fish kill on the River Roe near Limavady last July.
However, Jack McFarland, regional manager with the Water Service, said most incidents were of a minor nature.
He said the Water Service was third highest when it came to pollution, with industry second and farming at the top of the list.
"These are figures we are not very proud of," he told the BBC.
"We have hundreds of pumping stations, hundreds of sewerage works. Not everything works exactly to plan, as we would wish. We do have failures and we are not proud of them.
"But in fairness they are not causing any great degree of pollution; it is manageable."
A new water company will be formed in April
The Department of Regional Development confirmed that the Water Service caused 1,527 pollution incidents between 2000 and 2005.
These included waterways being polluted with raw sewage.
The Church Brae Burn near Derry was found to be "grey with fungus" and "septic" for six weeks last summer.
Mr McFarland said in the north-west area over the last five years there were 291 incidents, with only one of those deemed high, 5-6% deemed medium and the others low.
"The impact on the environment from our activities has been very low," he said.
"The quality of our infrastructure in some areas would probably be the major cause.
"Most of our sewerage depends a lot on pumping stations. We have power failures... but they don't last for long.
"But it's long enough for a sump to build up and maybe start to overflow - that in itself will create an incident. We do have pump failure and of course there are times when we do get things wrong ourselves."
Currently the Water Service is not prosecuted over such incidents, but Mr McFarland said Crown immunity would cease after it becomes Northern Ireland Water Ltd in April.