New Zealand's Department of Conservation (DOC) officers have had to kill 48 pilot whales that were among a pod of 120 whales beached in a remote part of New Zealand's 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 were flown to the scene, and found the whales in water, but with the tide receding.

One of the officers, Brent Beaven said half the whales were already dead, and the others would have soon perished in the hot, dry conditions, Radio New Zealand reported on Monday.

With the tide out and little hope of floating the survivors for up to 12 hours, the decision was made to put down the survivors, he said.

The whales will be left to decompose naturally on the beach, due to the remoteness of the site.