A mother and calf thrilled locals and tourists at the weekend as they swam near Aramoana and Deborah Bay, among other locations.
Department of Conservation biodiversity manager Jim Fyfe said he could not remember the last time a calf was seen in the harbour.
There were three or four notable sightings of calves being born ''haphazardly'' around the coast of New Zealand in the past decade.
Yellow-eyed Penguin Trust researcher Trudi Webster, who used to study the mammals, said southern right whales being born off the coast ''did not happen all the time''.
''It certainly used to be the case that whales used to be off the coast there. Unfortunately we dropped them down to such low numbers.''
It was likely the calf was born off the coast of New Zealand as the animals had an ''off-shore, on-shore'' migration rather than the north to south movements of some other whales, such as humpbacks.
Pre-whaling numbers of southern right whales were estimated to be 27,000, but were reduced to a low of less than 100 whales in 1925.
A 2013 estimate was about 2200.
