The beached long-finned pilot whales (Globicephala melas) were found by a hiker, who alerted a Department of Conservation field office, but almost half of the animals were dead by the time authorities could reach the site.
Department spokeswoman Gail Wright said there was "a great mystery" surrounding the mass stranding of the animals scattered along the beach.
The whales were in a miserable condition, and there were not enough resources to try and re-float what was left of the two pods, so it was made a decision to put them down, Leppens said.
He said the remaining whales were swimming about 400 meters (437 yards) from the shoreline on Tuesday afternoon and would continue to be monitored until they swam into deeper water, their natural habitat.
Since the beach is isolated, it's unclear how long the creatures had been stranded before they were discovered.More news: The Cavaliers Have Traded Kyle Korver To The Jazz
Marine Biologist Jeff Weir told Neil we could still be decades away from understanding why these animals beach themselves.
More than 300 people - including DoC staff, locals, schoolchildren, iwi and whale rescue groups - descended on Rarawa Beach to help release the eight whales yesterday.
"They don't hold much hope for them overnight, so I guess this morning we'll fly the vet in and a couple of specialists and we'll do an assessment on those four remaining pilot whales".
"Walkers on this remote section of the Wilderness Coast will also be advised not to enter the water due to the increased likelihood of sharks in the area attracted by the dead whales", an environment department statement also reads.
Half the animals were already dead and the rest were put down because there was no chance of rescuing them from the remote location United States travel blogger Liz Carlson was among a group of hikers on Stewart Island that found the whales and she described her frustration at being unable to help the huge sea creatures. "More than one factor may contribute to a stranding".