More than 6000 salmon were freed into the Ohau Canal near Twizel costing a nearby salmon farm more than $150,000.
The owners of the High Country Salmon farm suspect the fish that were released over the weekend was sabotage.
Staff made the discovery on Saturday morning and suspect someone entered the farm during Friday night and cut the ropes that held fish pen up.
"I noticed a lot of small fish on the side of the pens and I looked down into then pen and there was the net hanging half down in to the canal so I was pretty devastated," says High Country Salmon Worker Josh Wyber.
The salmon which weigh about 700 grams at the moment would have been harvested early next year.
"They're worth about twenty thousand to this business at the moment but in a years' time they'd be worth about a $150,000."
Mr Wyber says since the incident, the family run business has installed security cameras and an entry gate.
Police are investigating, and are keeping an open mind as to who may have let the salmon go.
"If this is done intentionally and someone's got an issue with a salmon farm or if they're wanting or just opened it up so lots of people can catch salmon it's not good enough, its criminal," Oamaru police Sergeant Tony Woodbridge says.