Marlborough trucking firm Broadbridge Transport has been fined fifteen-thousand dollars after its wash water residue contaminated a creek in Renwick.
Several eels died when the water containing crushed grapes and grape waste was discharged into School Creek last April.
Residents reported that the creek water turned orange and had a strong sulphurous odour.
Marlborough District Council testing showed it was severely depleted of oxygen, significantly effecting its ecology and resulting in the deaths of eels.
Council said the company allowed its truck operators to wash their vehicles during last year's grape harvest at its Renwick facility that was not designed for large volumes or contaminated water.
It flowed into a roadside drain that was discharged into School Creek.
Council consents and compliance group manager Gina Ferguson said Broadbridge was willing to cooperate after pleading guilty to the offence under the Resource Management Act.
"This was a restorative justice process. The company was open and willing to cooperate from the beginning, and the severity of the penalty was reduced because of their good actions."
It resulted in the company committing to environmental improvement measures for School Creek over the next three years.
"I believe they underestimated the potential impacts of the discharge, but did what they could to put it right. They accepted responsibility and took ownership of the issue."
Ms Ferguson said she hoped the conviction would send a strong message to other businesses that fail to meet the requirements of the Proposed Marlborough Environment Plan and the Resource Management Act.