Northern Ireland Water has been fined £5,000 for causing river pollution in Co Armagh.
A sewage blockage caused by inappropriate items was blamed for the discharge from Carron Road pumping station, Crossmaglen, in April 2007, the company added.
A tributary of the Rathkeelan River was polluted.
A spokeswoman for NIW said: "NIW acted quickly to stop the discharge and to clean up the site. Further investigation found that the blockage had been caused by a large amount of inappropriate items, which the sewerage system is not designed to cope with.
"This blockage and subsequent sewage spill could have been prevented."
The company asked for the public's assistance by not placing inappropriate items in the toilet, down a drain or into the sewers.
The spokeswoman added: "Inappropriate items include cotton buds, nappies, sanitary items, household wipes and condoms which regularly make their way through the sewerage system and cause serious blockages, leading to the operation of emergency overflows.
"This can cause harm to property, wildlife and the environment."
NIW is investing £1 million per working day to bring water and wastewater services up to an acceptable standard and the spokeswoman said she was confident that when this investment programme is complete, such incidents will become less common.
The case against NIW was heard at Newry Magistrates' Court.
A spokesman for the Northern Ireland Environment Agency (NIEA) said: "A sample taken at the time of the incident confirmed that the discharge contained poisonous, noxious and polluting matter which would have been potentially harmful to fish life in the receiving waterway."