Thousands of small salmon died after a farm in south Westland was burgled on Saturday night.
Thirteen-thousand of the fish suffocated when the water supply to the salmon rearing tanks was shut off, at the Paringa Salmon Farm.
Co-owner Alice Bellio says the babies were going to be next year's production.
"There was no water running through the tanks and no oxygen, therefore all the fish died," she told Newshub.
Ms Bellio says they're gutted by the loss and it's going to come at a huge cost to the business.
"It's basically six months of work put into raising those fish from eggs to the size that they were," she said.
"[It's costing] probably between $15,000 and $20,000."
Whoever tampered with the equipment knew what they were doing, Ms Bellio says, as there was no other damage nearby. The net used to catch fish was also thrown into the pond.
A vehicle parked on State Highway Six, several kilometres up the road from the farm's grounds, was also tampered with.
Nothing was taken from the farm during the incident.
"It's cruelty, basically - when we saw those fish dead at the bottom of the tank in the morning, when we lifted the lid, it was just really upsetting," Ms Bellio says.
"It's a shame that people would do that - I struggle to understand why."
Police ask anyone with information to contact them on (03) 750 0850, or anonymously via Crimestoppers on 0800 555 111.
Newshub.
