An investigation conducted by the Otago Regional Council (ORC) has determined that a faulty submersible pump electrified water in Mill Creek near Arrowtown, killing a number of trout in the stream.
Multiple fish deaths have been reported in the area in recent weeks and the ORC has ruled out poisoning or disease as a cause after autopsies on the dead fish were carried out.
At least 20 trout were discovered dead over several days in June.
The ORC says the submersible pump was found in a privately owned wetland adjourning the Mill Creek stream and that a comment made by Fish and Game suggested "something" was causing a magnetic field attracting the fish.
Investigators then realised that most of the dead fish had been reported in the vicinity of the pump's intake.
Upon inspection from an electrician, they found the pump had a faulty float switch, sending an electrical current directly into the water and electrocuting the fish.
The owner of the pump arranged to have it removed immediately, as soon as the fault was identified.
Further water sampling in Mill Creek had found no high levels of chemicals or heavy metals except aluminium in the vicinity of the pump.
"It's a relief to be able to confirm that water pollution was not a factor in these deaths and that there is a simple explanation and an easy solution in this case," ORC's director of operations Scott MacLean said.
Because the pump wasn't operating all the time, the fish were being affected sporadically, which limited the number fatalities.