At least 13,000 salmon died on Saturday night, after their water supply was shut off by intruders on a salmon farm.
South Island police are investigating the incident that occurred at the Paringa Salmon Farm Café, South Westland.
Overnight on December 3, important machinery was tampered with; the alleged offenders shutting off the water supply to the salmon rearing tanks, causing the fish to suffocate due to lack of oxygen.
Haast police station's senior constable Paul Gurney said it had caused thousands of dollars of loss for the business.
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A vehicle parked several kilometres up the state highway was also tampered with at the same time.
Police said they would like to hear from anyone who saw anything suspicious in the South Westland area during the time of the incident.
Anyone with information is asked to contact Haast police on 03 7500850 or email Paul.Gurney@police.govt.nz.
Information can also be provided anonymously via Crimestoppers on 0800 555 111.