The trench had caused flooding of the Hokio A Block, which also shares a border with the block the lake is on, affecting the land another farmer leased - obliterating the paddocks he usually used for calving, and depositing silt and sand across the area.
A source close to the affected farmer said the lake had been an important home to eels and fish, and it had been possible to see the small fish trying to get back into it as they were being washed out through the trench.
"We're all here for the same reason but we don't go around destroying 1000-year-old lakes just because our paddock has got water in it," the source said.
Horizons Regional Council had been notified and confirmed it was investigating, but would not comment further at this stage.
Mr Jacobs said he was upset that the farmer who dug the trench had told Horizons that Mr Jacobs had given him permission, which was not true.
"[The farmer] had no permission or directive from us to dig that drain onto the Hokio A Block," he said.
"[Hokio A] is not my land. We didn't even know he'd done it. I said after it wasn't a bad thing the lake's water level was brought down, but he never had permission."
Both the source and a local iwi member told The Horowhenua Chronicle the Hokio A Block contained historical middens. However Phillip Taueki, believed to be the chairman of the trust that owns the Hokio A Block would not confirm this, and became abusive when asked for a comment.
Mr Jacobs said the trust's land was leased to farmers who had cows "trampling around all over it" anyway.
The farmer alleged to have dug the trench could not be reached for comment by time of print.