Some 97 whales and three dolphins died in the stranding, which conservation department staff were alerted to around midday local time on Sunday.
“Only 26 of the whales were still alive at this point, the majority of them appearing very weak, and were euthanized due to the rough sea conditions and almost certainty of there being great white sharks in the water which are brought in by a stranding like this,” biodiversity ranger Jemma Welch said in a statement.
Pilot whales — small, toothed whales with a bulging forehead, a short snout and pointed flippers — are sociable creatures, and live in groups of dozens, hundreds or even thousands.
Two more whales were stranded on Monday and also had to be euthanized, the Department of Conservation said, adding that the whales will be left to decompose naturally.
Representatives from the Indigenous Hokotehi Moriori Trust and Ngāti Mutunga o Wharekauri Iwi Trust performed a karakii/karakia — a prayer, or incantation — to honor the spirit of the whales on Sunday, the department added.
Mass strandings are common on the Chatham Islands, according to the department, which said that up to 1,000 animals died in a stranding in 1918.