Twenty-three seals have been clubbed to death in a "cowardly and callous" attack at the Ohau Point seal colony north of Kaikoura.
Eight newborn pups, 13 females and two bulls were killed.
The Department of Conservation (DoC) says it appears the seals were killed over separate days, at least a week ago, and describes the attackers as cowardly and callous.
[image:675:half:right]A senior lecturer in zoology at Otago University, Dr Bruce Robertson, put the actual death toll at 49.
He says the 13 females would have had pups totally dependent on their milk for survival and would have also been carrying a pup, as they mate about a week after giving birth.
Matt Chambers, the manager of tourism venture Seal Swim Kaikoura, says the timing could not have been worse.
"It's a breeding colony that they targeted and ... right now ... is when all the mothers are giving birth so they've picked on an area at its most vulnerable time. There'll be pups that have been abandoned by the mothers as they've fled from the area."
Mr Chambers says the colony is easily accessible by vehicle and then foot, but that could now change.
He says Ohau Point is the largest breeding colony on the east coast of the South Island and needs greater protection.
Listen to Matt Chambers
Carnage shocking - department
DoC South Marlborough area manager, Dave Hayes, says it was shocking to discover the carnage.
"The fact that people presumably returned to the site to carry on with more killings indicates to us that it was quite a callous and concerted activity."
The department says the attack at the colony, 22 kilometres north of Kaikoura, appears to have been carried out by one or more strong people, using club-like weapons such as bats.
DoC says it would have taken a person or people of some strength to do this damage
Mr Hayes says some of the female seals that were killed had pups, and they will starve to death without their mothers' milk.
New Zealand fur seals are protected and people found guilty of harming them can incur a maximum of six months' imprisonment or a $250,000 fine.
The department is asking anyone with information to contact its South Marlborough area office at Renwick, or police.