It may never be known why a pod of pilot whales repeatedly restranded at Golden Bay this week, leading to the death of 82 of the mammals, a whale expert says.
The whales were part of a pod of 99 which beached in the Farewell Spit area about noon on Monday.
Volunteers twice refloated the surviving whales only to have them restrand, with more dying or having to be put down after each attempt.
Department of Conservation (DoC) area manager John Mason said the remaining 33 whales were shot in the head with a high calibre rifle shortly after the decision to put them down at 8.15am today.
Only 17 whales had successfully refloated in the past three days.
Whale expert Anton van Helden said whales stranded themselves as a social survival response.
"So they work together to look after one another, so if there are animals that are struggling there may be other animals coming around trying to support them and they're getting into trouble themselves.''
Whales were conscious breathers, meaning they would be aware of the concept of drowning, Mr van Helden said.
"Animals strand themselves essentially so that it doesn't have to work so hard to keep its blow hole out of the water...so basically stranding is a survival response.''
The area where this week's stranding happened was a classic "whale trap'', he said.
"You've got a very low-grade beach so echo location isn't functional. It essentially can look like open ocean.''
Mr van Helden said while it was hard to know exactly what was going on, it was not surprising the whales restranded.
"Animals who have already been pretty disrupted by probably the initial event and are looking to sort themselves out socially, and they find themselves in trouble again, and it's not really that surprising.
"Pushing whales out to sea does not necessarily mean they survive, it's very hard to monitor what goes on afterwards,'' Mr van Helden said.
There were so many variables associated with whale strandings it was impossible to say why this particular group repeatedly restranded, he said.
It would be impossible to pin down why this restranding had happened bar tests being done on the carcasses and showing they had something such as a virus.
"... so to speculate on the specifics of why a whale has stranded is fruitless,'' he said.
However, it was common for a pod of whales to restrand a couple of times.
"People are looking for some extreme reason [for the stranding] but it's very hard to say, and it's not that we don't know why whales strand, we do, we don't necessarily know what particular thing was a trigger in a particular case.''