Northern Ireland Water (NIW) has been fined over £2,300 for two instances of polluting a waterway.
The case, heard at Downpatrick Magistrates' Court on Wednesday, heard that "poisonous" discharge was found in the Drumaness and Ballynahinch Rivers.
Magheratimpany sewage pumping station and Drumaness Wastewater Treatment Works were traced to be the source. Both are owned and operated by NIW.
It is NIW's fourth fine for pollution since July.
In June 2009, a water quality inspector discovered a plume of grey sewage for approximately 100 metres and significant sewage fungus growth in the Drumaness river.
The Magheratimpany sewage pumping station, owned and operated by NIW, was found to be the source of the pollution.
The inspector visited the Drumaness Wastewater Treatment Works operated by NIW in December 2009 and January 2010.
Sewage fungus was found on the first visit and a discharge of untreated sewage on the second occasion.
Samples taken at both the pumping station and the treatment works confirmed that the "discharges contained poisonous, noxious or polluting matter which was potentially harmful to fish life in the receiving waterway".
A spokeswoman for NI Water can said the incident at Magheratimpany Sewage Pumping Station was of low environmental impact as no fish were killed.
She said the incident occurred "when the duty pump failed and the stand by pump did not activate".
"NI Water was alerted at 12:00 and by 13:14 the discharge had ceased and staff had the pumps operational again", she said.
"NI Water treats any issue of pollution extremely seriously and has taken steps to ensure a similar incident at Magheratimpany Sewage Pumping Station does not happen again."
The spokeswoman said NIW had invested almost £500m over the last three years to bring the sewerage network system and wastewater treatment works up to an acceptable standard.
She said the incident in the Drumaness River was linked to a capacity problem at the Drumaness Wastewater Treatment Works which meant it was unable to cope with extra flows received during the period in question.
"While a number of remedial actions were undertaken by NI Water prior to this incident, there were no quick solutions to this problem. It required substantial capital funding and consultation with relevant bodies," she said.
NIW has also received a number of other fines over the summer.
In August it was fined £1,000 for polluting a tributary of the River Lagan, at Lock Keepers Lane, near Shaw's Bridge in Belfast and another £1,000 for two offences of polluting a waterway at Waringstown Stream.
In July, NIW was fined £1,000 after a stream beside a wastewater treatment works in Greysteel, County Londonderry was found to be visibly polluted.