Something fishy is going on in Twizel, after 6000 salmon were illegally released into a waterway.

A salmon farming company has lost more than $150,000 worth of salmon after someone cut a net, releasing 6000 undersized fish into the Ohau Canal, on Friday night.

Police are investigating the incident at High Country Salmon, near Twizel.

Senior Constable Nayland Smith, of Omarama, said it was the first time he had dealt with such an incident, calling it a "malicious act".

JOHN BISSET / FAIRFAX NZ An aerial view of High Country Salmon, on the main road from Twizel to Omarama.

He said whoever released the salmon did it "on purpose".

High Country Salmon farm manager John Jamieson said the cut net and ropes were discovered on Saturday morning.

He estimated the release would cost the business $150,000. The salmon were not insured.

High Country Salmon High Country Salmon, in the Mackenzie Country.

"It has left a hole for us, but we'll manage," he said.

Jamieson said a lot of time and effort had gone into breeding the salmon, which had been on the farm since October. They would have been harvested early next year, when they were about 3 kilograms each.

Some staff had "a real emotional attachment to the fish", he said.

He said staff would be at work from 5.30am and would often be there feeding the salmon until 10pm to give them "the best chance we could give them".

In five minutes someone had thrown "eight months of work down the gurgler", Jamieson said.

It was "unlikely" High Country Salmon would see the salmon again as a condition of their fish farming licence meant once fish were released into the canal they were owned by Fish and Game.

Central South Island Fish and Game chief executive Jay Graybill said once fish were released into "natural water" they were under the regulation of Fish and Game.

He labelled the incident "a senseless act of vandalism and Fish and Game cannot condone it in any way, shape or form".

Graybill warned anglers the released fish "were on average about 15cm long, undersized and a breach of the fishing regulations". The regulation size of salmon was a minimum of 30cm in length.

"We will be actively ranging the canals and anyone found with undersized salmon will be issued with an infringement notice," which carried a maximum fine of $5000, he said.

However it would be "virtually impossible" to catch the fish as there is "so much water in the canal".

Jamieson said farm employees could not believe their eyes when they saw the thousands of freed salmon in the Ohau Canal.

"They were jumping around like dolphins – and that is something you never want to see," he said.

"Whoever has done that knew what they were doing."

The 17-year-old business was a "local, niche outfit" that did not want to put barbed wire fences around the property.

"We have taken for granted that nothing like this would happen."

The company has since installed cameras. The firm ordered them months ago but they "unfortunately were not high on the priority list", Jamieson said.

It was too early to speculate on who may have cut the nets.

Senior Sergeant Trevor Thomson said the live salmon were released "on purpose".

"They don't know who it is at the moment."

Anyone with information was urged to contact police, or Crimestoppers on 0800 555 111.