Department of Conservation officers have had to kill 48 pilot whales that were among a pod of 120 whales beached in a remote part of Stewart Island on Sunday.
The whales were discovered at the southern end of Mason Bay by two overseas visitors, who raised the alarm.
Two DOC officers flown to the scene found the whales in water, but with the tide receding.
One of the officers, Brent Beaven, says half the whales were already dead, and the others would have soon perished in the hot, dry conditions.
He says with the tide out and little hope of floating the survivors for up to 12 hours, the decision was made to put them down.
DOC southern islands area manager Andy Roberts says the decision was difficult but was based purely on the welfare of the animals.
The whales will be left to decompose naturally on the beach, due to the remoteness of the site.