Forty-two yellow-eyed adult penguins have been found dead along Otago Peninsula in recent weeks.
The Department of Conservation found them at breeding sites during nest checks.
Biotoxins may be involved, but DoC biodiversity programme manager David Agnew said on Tuesday it is extremely unusual, as no other marine life along the peninsula seem to be affected.
Massey University pathologist Stuart Hunter has examined a dozen of the penguins and says testing so far has been inconclusive.
He said the good conditions of the penguin bodies mean they must have died rapidly.
Mr Hunter suspects biotoxins could be involved but he says it would be strange as no other fauna appear to have been affected.
He says the stomach content tests, when they are completed, should reveal more.
Results of tests at Massey are due by the end of the week.
