It is believed large swells likely resulted in more than 40 fur seals or kekeno washing up dead on a remote bay in Banks Peninsula, Canterbury.
About 25 were found dead yesterday afternoon at Te Oka Bay.
The Department of Conservation examined the area today and said locals had told rangers that seas were stormy and heavy about the time they think the seals died.
Mahaanui district operations manager Andy Thompson said the beach was very rocky and it was likely the seals were thrown up by the swells and on to the shore.
Mr Thompson said the seals appeared to have been dead for about two weeks.
He said only one adult seal was killed, while the rest were pups at weaning age.
He said the bays didn't have a lot of protection, so it was not uncommon to find young seals that had been thrashed by the seas.
"Those bays are all south facing bays so there's no protection. The southern swells sweep right in there and it's not uncommon that we do find seals getting clobbered by some of those heavy seas. Adult seals can deal with it fine but young seals don't always have the stamina or strength to deal with those heavy swells."
Andy Thompson said they had sent six of the seals to Massey University to investigate the cause of death and expected the results to come back within a week.
Kekeno were the most common seals in New Zealand, and about 12,000 lived around Te Oka Bay.