An adult seal and 40 pups have been found dead on an isolated part of Banks Peninsula near Christchurch.
Department of Conservation Christchurch operations manager Andy Thompson said the agency was alerted to the deaths late yesterday and was "keeping an open mind" about how they died.
"Our ranger counted 41 dead seals. One was an adult seal and the rest were pups at weaning age," he said.
"We think they had been dead for about two to three weeks by the state of the bodies. They were found high up on the beach, above the high tide mark, and we think they were probably swept there by high seas.
"This is a remote area of the peninsula and locals have told us that there were stormy, heavy seas about the time we think the seals died.
"We hope to send some of the dead seals to Massey University for necropsy to investigate the cause of death. We are keeping an open mind but at this stage, given the circumstances, we are leaning towards natural causes."
The department will keep a monitoring brief on the fur seal (kekeno) population. The results of the post mortem examinations won't be known for at least several days.
Around 12,000 kekeno live and breed around the Te Oka Bay area. Kekeno are the most common seals in New Zealand and their population of more than 200,000 is growing.