The dead and dying pilot whales were discovered by hikers on Tuesday night on Stewart Island, 30km south of the South Island, but it was two days before they were able to raise the alarm with conservation rangers.
Ranger Phred Dobbins was one of two staff flown to the site at Doughboy Bay on Thursday.
"Eight of the whales were still alive when we arrived. However, refloating them was not an option given the length of time they had been stranded in hot, dry conditions.
With the tide well out, we saw little hope of keeping the animals alive until enough rescuers could be flown in to assist," Dobbins said.
The whale carcasses will be left to decompose naturally on the beach due to the remoteness of the site.
Whale strandings are not uncommon in the area, Dobbins said, with conservation staff dealing with a stranding at least once every couple of years.