"And then there was 12 animals that stranded in the 1980s in the Chatham Islands and they all died before they were found. We have no idea what caused that either.''
When small groups or single orca strandings occurred, it was often related to them hunting for rays but it was impossible at this stage to say whether that was the reason for this stranding, Dr Visser said.
"They don't generally strand and when they do strand they don't mass strand like this. This is a very unusual situation.''
The Department of Conservation described the latest stranding as a blow to New Zealand's small orca population.
The whales were seen swimming in shallow water at Te Waewae bay near Tuatapere yesterday evening and by midnight had beached at Blue Cliffs beach, DoC spokesman Reuben Williams said.
When DoC staff arrived at 1am, the eight adults and one juvenile whale had died, he said.
"In terms of endangered species, there's only 200 in the local population. Nine out of 200 - it's a bit of an impact on the population and it's unfortunate.''
DoC had consulted local iwi before removing one of the whale carcasses and taking DNA samples for research purposes, Mr Williams said.
No other whales had been seen swimming in the area.
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