Sewage is likely to have caused the deaths of dozens of eels in a Picton stream, the local council says.
The Department of Conservation counted 24 dead eels in the stream, and noticed dozens more.
That followed the discovery earlier this year of hundreds of dead eels in a creek near a Blenheim vineyard.
Tests has failed to determine the cause of death in that case.
Marlborough District Council said human effluent from an incorrectly connected sewerage pipe from a new house had been identified as the probable cause of the latest deaths.
Council operations and maintenance engineer Stephen Rooney said it appeared an error had been made by the drainlayer, who had accidentally connected the household wastewater to the stormwater system, which discharged into the stream.
It was not clear if the eels died as a result of raw effluent, or from a household cleaning product flushed into the stream, Mr Rooney said.
"For example, if you're using an ammonia-based toilet cleaning product, that on its own may have been sufficient to cause the environmental damage."
The drainlayer had now fixed the problem, he said.
He had seen the same mistake made in the past.
"I've seen it occur where both sets of pipes are very close to each other and they're accidentally cross-connected, but cases of this happening are very infrequent."
The council would look at ways of reducing the chance of a similar mistake being made again, Mr Rooney said.
That could include colour coding, specifying greater distances between outlets, or more robust checking when a connection is made.
The incident also underlined the importance of ensuring that only rainwater was going down stormwater drains, he said.
"Household cleaners, weed killer and paint, even the detergent from your car wash - they can all be toxic for the inhabitants of a small stream."