Dog owners have been warned to keep their pets away from weak, skinny fur seals seen searching for food along the country's eastern coastline.
The Department of Conservation says unusually high numbers of emaciated seals have been noticed on beaches, and callouts for sick and dead seals are on the rise.
Marine adviser Laura Boren says more seal pups are being found dead, particularly around Kaikoura and Otago, as colonies struggle to find enough food.
The department was closely monitoring the situation but advised people to keep their dogs away from the newly weaned young pups which have started showing up on beaches.
"They're just learning to make it on their own and it's normal for many to be emaciated and weak," Ms Boren said.
"With more in poor condition this year, it's important dog owners control their dogs around seals."
She said they were small, weak and helpless, and be could easily picked up, shaken and killed by a dog.
The department urged the public to leave seals alone unless they were being harassed by dogs or people, entangled in marine debris, severely injured, or on a road or private property.
Tests were under way on a selection of the dead pups to find out more about the deaths and understand the extent of the situation, Ms Boren said.