Two livestock companies have been fined more than $70,000 each following a discharge of animal effluent into the Makino Stream.
PGG Wrightson and Carrfields Livestock appeared at the Environment Court in Levin on Monday.
The offending occurred at the Feilding stock yards, which is the largest stock yard in the southern hemisphere, in February last year.
Effluent had entered the stream through a storm water pipe connected back to the stockyards.
The discharge was brought to the attention of Horizons Regional Council by a member of the public.
Horizons strategy and regulation group manager Dr Nic Peet said the council takes its role as an environmental regulator seriously and puts significant matters before a Judge.
"The Court noted that PGG Wrightson and Carrfields are large companies who had been very careless about how the managed disposal of animal effluent.
"The Judge commented that commercial operators should be aware of their legal responsibilities and manage their businesses accordingly."
Dr Peet said the companies guilty pleas were a useful acknowledgement of the offending which reduced the time and cost of prosecution.
Judge Dwyer handed down fines of $75,000 to PGG Wrightson, and $71,250 to Carrfields Livestock.
Dr Peet said another significant Environment Court prosecution was adjourned, and four further investigations are underway.