University of Otago zoology professor Liz Slooten said the dolphin was of the "common dolphin" species, as identified by its "almost yellow pattern" along its sides.
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These dolphins spent most of their time further out to sea than other species, such as bottlenose and Hector's dolphins, which were often found near beaches and estuaries.
But common dolphins often made their way into Wellington Harbour, with a pod spotted around Evan's Bay just on Friday evening.
Unlike the highly social whales, which often stranded in large pods, dolphins were more independent, and when sick or injured would depart from the group.
From the information at hand and no clear injuries it was likely the dolphin was sick or elderly, Slooten said.
Other potential causes included ocean predators like orca; or human factors, such as being caught in fishing gear, a seismic survey affecting its hearing, or a boat running it over.
Several people had also commented on social media that the death could be linked to a wastewater overflow days before Christmas, when an estimated five million litres flowed into the harbour.
Slooten said that was a lower possibility, but would only be determined after a proper autopsy had been carried out.
The sighting comes amid a summer of multiple sighting and strandings of marine life.
Yesterday more than a thousand people rushed to Matarangi Spit on the Coromandel to try and save a pod of 11 pilot whales that stranded.
Four died, but rescuers managed to refloat seven and guide them out into deeper waters.
A DoC spokeswoman said on Sunday there had been no restrandings.
Several sharks have also been spotted close to shore already this summer, including an endangered great white shark that was killed after getting caught in a fishing net near Orewa Beach.
Slooten said while there was "quite a bit" happening in the marine sphere, it was not unusual for this time of year.
"It seems to be a combination of a few factors. This time of year many species come closer to harbours and the shore over summer, often as they chase food, fish species that also come closer to shore.
"And also at this time of year everyone is out on their boats and at the beach, so not only is there more action with potential dolphin and whale strandings, but more people around to notice it."
