Horowhenua District Council has admitted dumping millions of litres of sewage into a stream that feeds the Manawatu River.
It pleaded guilty on Monday during at the Palmerston North District Court, to a charge of illegally discharging sewage in 2007.
The council had no resource consent to release the waste from its effluent ponds in Shannon into the Mangaore Stream.
Manawatu Regional Council brought the prosecution. It says 8,553,200 litres of raw sewage were released over about two days.
Two contractors employed by the Horowhenua council are also in court over the incident.