Council-owned flood water pumps nationwide are killing almost all the native eels that pass them through, according to a report obtained by Newshub.
The information - obtained by Official Information Request by Forest and Bird - highlights a major threat to the survival of our already threatened eel populations.
Horrific images show what happened to longfin and shortfin eels that travelled through the blades of axial-type water pumps.
"What the original designs that are currently in place do, which is effectively, it sort of dices them up," said Dr Bruno David at Waikato Regional Council.
Waikato Regional Council tests have shown in some cases all the adult female eels are killed.
The females - some up to 100 years old - migrate out to sea during heavy rains in Autumn, heading for the Pacific Islands to breed for the sole time in their lives.
NIWA eel expert Dr Don Jellyman says it's a worst case scenario.
"You've kind of got the perfect storm there of a problem," he said.
"Pumping stations cranking up, eels trying to migrate out and the chances of those females surviving passage through a pumping station is virtually zero."
Longfin eels are one of the largest of their kind in the world, with some females growing to more than one and a half metres long, and it's because of this that when they pass through a flood water pump they don't stand much of a chance.
"Eel are already facing a lot of threats with pollution and wetland drainage - that's an important habitat for them - so to find out that these pumps are also killing them it's putting a lot of pressure on the survival of the species," said Forest and Bird's freshwater advocate Annabeth Cohen.
Waikato Regional Council operates more than 120 of the deadly pumps but is now testing an eel-friendly version.
"Last week we had 30 large eels go through that, the previous pump station which was there, which resulted in 100 percent mortality of those eels," said Dr David.
"This case we've had 30 go through and 29 of those have come through alive."
But Forest and Bird says there could be a thousand pumps around the country and it wants the Department of Conservation to work with councils to fix the pumps.