On Friday night support ropes around a net at High Country Salmon were cut and about 6000 salmon released into the Ohau hydro canal and Lake Ruataniwha, business owner Peter Logan said.
Mr Logan said the vandalism meant the loss of thousands of dollars in sales.
The value of the fish was estimated to be about $20,000, police said.
It was the first time the business had been attacked in that way in the 17 years it had been running.
Farm manager John Jamieson said the 15cm-long salmon had been in the pen since October.
He said the early release could be the work of an angler, but if that person thought they were doing anyone a favour they were wrong.
The sudden drop in numbers could impact the numbers of staff brought in to help with the upcoming harvest, the lost revenue could affect future growth projects at the farm, and the introduced salmon could take out some of the natural food source native animals relied on, he said.
"There are only so many fish the ecosystem can support.''
``The guys start at 6am and stay until 10pm to give them care ... it has been a real kick in the guts.''
As a result of the weekend vandalism security cameras and lights have been installed.
``It was a shock. It was something we didn't expect. We didn't have the security you put up in a big city.''
Fish and Game spokesman Jay Graybill condemned the vandalism.
``All anglers should consider it a betrayal of all people who like to catch fish.''
To be a legal catch, a salmon must be 30cm long.
Mr Logan urged the public to come forward with any information which could help the police in their investigation of the attack.