The seals - some of them newborn pups - were found bashed to death at the Ohau Point seal colony near Kaikoura. A number of other seals were injured.
Ohau Point is a popular tourist spot for viewing fur seals and is visited by thousands of people each year.
Forest & Bird marine conservation advocate Kirstie Knowles said the organisation was disgusted and disappointed by the barbaric killing of the seals.
"The deliberate killing of our wildlife is disgusting and the people of Kaikoura and New Zealand will be outraged," Ms Knowles said.
She said Forest & Bird was supporting the Department of Conservation (DOC) in its pleas for anyone with information about the cowardly attacks to come forward.
DOC Nelson Marlborough spokeswoman Trish Grant told NZPA the seal colony was right beside State Highway 1 and there may have been witnesses.
"The public are going to be really important. What we want to do is find the people responsible as soon as possible," Ms Grant said.
DOC contractors found the seals late last week and the condition of the dead seals suggested the attacks had been carried out over separate days, over the past week or two, DOC South Marlborough area manager Dave Hayes said.
"This is a callous and cowardly attack on seals, especially newborn pups, unable to defend themselves against someone striking them with a bat or similar club-like object," Mr Hayes said.
DOC believed someone of some strength carried out the attacks.
Senior lecturer in zoology at the University of Otago, Bruce Robertson, said the real number of seal deaths would likely be higher than the 23 reported.
"This is because females at this time of year are most likely to have a pup and these pups are totally dependent on their mother's milk for survival. So if the mother is killed the pup will die," Dr Robertson said.
The females, which mated a week after giving birth, would have also had the developing embryo of next year's pup in the womb, which would have also been killed, he said.
All up the total loss of life would likely add up to 49 fur seal - 13 females, 13 dependent pups, 13 developing embryos, plus the eight pups and the two males, Dr Robertson said.
Anyone who kills or harms a fur seal or other marine mammal faces penalties of up to six months' jail or a fine of up to $250,000 plus a further fine of up to $10,000 for every marine mammal the offence is committed against.
Last month Harley David McKenzie, 20, of Tuatapere, became the first person to be jailed for attacking a leopard seal when he was sentenced to four months in prison for taking and injuring a leopard seal in Southland's Te Wae Wae Bay last year.
