Twenty-three seals, including eight newborn pups, have been found clubbed to death on the Kaikoura coast.
The bashed seals were found on Wednesday by Department of Conservation contractors at the Ohau Point seal colony, about 22km north of Kaikoura township.
Some of the pups killed were just a few days old. The dead seals also included 13 females and two bulls. Other live seals had injuries suggesting they had also been struck, the Department of Conservation (DoC) said.
New Zealand fur seals are protected by law and it is an offence to kill or harm them.
Conservation Minister Kate Wilkinson called on anyone with information on the attack to contact authorities.
"I'm beyond appalled," Mrs Wilkinson said.
"This isn't just a random act of violence. To go around and club 23 seals to death over a number of days is very deliberate and you have to question the state of mind of someone who can carry out such a cruel and abhorrent attack."
She said DOC was determined to catch the offenders.
"I call on the public to help out. If you know who did this or have suspicions then turn them in. New Zealanders hate this sort of behaviour and we need to send a message that we're not going to put up with it."
Under the Marine Mammals Protection Act 1978 there are penalties of up to six months' imprisonment or a fine of up to $250,000 for killing or harming fur seals or other marine mammal plus a further fine of up to $10,000 for every marine mammal the offence was committed against.
DoC South Marlborough area manager Dave Hayes said the condition of the dead seals suggested whoever was behind the seal attacks had carried it out over a number of days.
"Some of the dead seals appear to have been killed a week ago; others look to have been killed around a week or so beforehand.
"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.
"The smashed skulls of several dead seals we examined suggest they may have died in one forceful blow to the head indicating it was a person or persons of some strength who carried out the attacks."
Seal pups from the Ohau Point colony have in recent recent years been hanging out at the Ohau Stream waterfall pool between April and November, while their mothers head out to sea. Between 100 to 200 pups have been seen up the stream at the pool.
Mr Hayes said Ohau Point was visited by thousands of people every year and visitors enjoyed watching seal pups in the nearby Ohau Stream.
"It is deeply disappointing to see such a lack of respect and appreciation for these captivating marine mammals that are an important part of Kaikoura's internationally-renowned marine wildlife tourism," he said.
"The Ohau Stream waterfall pool has particularly become a major attraction being visited by around 5000 people a month in the eight months or so of the year seal pups from the Ohau Point colony gather there.
"People have great pleasure in seeing the young pups playfully cavorting at the pool. The eight young pups killed in this attack might have been among them in four to five months' time had they not been killed.
Mr Hayes said antagonism towards seals sometimes came from the "misplaced belief" they were competing for fish caught for human consumption.
"That isn't the case. Research on Kaikoura seals indicates the seals primarily eat lantern fish which are not sought after in fishing."
The Kaikoura District Mayor Winston Grey described the attack as "beyond belief".
He said the region's wildlife is an important part of the district's tourism industry.
"The town is busy with people - they're here to see our marine wildlife."
Mr Grey said the spot is a popular tourist destination and the majority of people know to keep their distance.
"They've been here for hundreds of years. Most people do not approach them."
A Department of Conservation spokesperson said attacks on seals were rare, with only three of four reported nationally each year. It was unusual to have this many animals killed, she said.
DoC recommends staying at least 20m away from seals at all times.
Last month Harley McKenzie, a 20-year-old Southland man with a history of violence and a previous conviction for animal cruelty, was jailed for four months for his role in an attack on a leopard seal in October last year in which fist-sized stones were thrown at the seal and it was dragged around a beach by its tail. Two other men were fined $5000 and $7000 respectively for their part in the attack.
In 2005, All Black Andrew Hore and friends Hamish Wilson and Matthew O'Connell were each fined $2500 after they shot at seals, killing one, on the Otago coast.
Prior to the arrival of Europeans about two million fur seals lived around the New Zealand coast, but in the early 1800s they were nearly wiped out in Kaikoura and other areas due to hunting. Seals were given full protection by the New Zealand government in 1849.