The pollution happened in March last year when the Killymoon River was polluted following a leak from the Sandholes Sewage Pumping Station.
NI Water said the incident was caused by ‘complications’ which led to a temporarily shut down and caused the discharge.
A spokesperson for NI Water said: “We responded quickly and completed repairs the same day.
“A review of the alarm also took place and changes have been made to ensure NI Water is alerted before an un-consented discharge to the watercourse takes place.”
NI Water was fined £500 plus £34 costs. in mitigation the firm said, throughout the process, it fully co-operated with the Northern Ireland Environment Agency.
Luke Walsh