The first, an adult female, was found on Sunday by the Department of Conservation at Blueskin Bay estuary after Doc was alerted by the public.
A newborn calf was discovered on Monday in the same area.
The second, another adult female, was found on Tuesday afternoon at Blueskin Bay.
Doc biodiversity programme manager David Agnew said until necropsies (animal autopsy) had been done by Massey University's wildlife centre, there was no way of knowing if the deaths were something to be concerned about.
''At this stage it could be coincidence. Three in a week, though, is far too many.''
One of the females was probably the mother of the dead calf and possibly died during the birth process, he said.
It was hoped the necropsies would identify cause of death and DNA testing could show which was the calf's mother.
The quick thinking of the public in reporting the dolphins to Doc meant the department had the best chance to discover the cause of death, as the bodies were not decomposed.
Hector's dolphins were found off the coast of Otago mostly from Taiaroa Head to Karitane and periodically washed ashore.