From 1998 until 2017, it was considered to be simply a New Zealand population of Cygnus atratus, until DNA recovered from fossil bones determined that it was indeed a separate species, much larger and heavier than its Australian relative.

They were found associated with moa and fish bones, seal hair, adzes, spears, and sinkers, indicating the swans were hunted by early Māori inhabitants of the cave.

By the 1950s, Forbes's type material was considered lost, so Oliver in 1955 declared C. sumnerensis a nomen nudum, and redescribed the species, using fossil bones from the Chatham Islands, as Cygnus chathamensis.

[10] In 1998, Worthy compared a large collection of C. sumnerensis from Marfells Beach near Lake Grassmere with Australian swan bones, but could not find any marked differences in size or proportion.

The researchers chose the Moriori name poūwa for the species, from a legend of a large bird that lived in Te Whanga lagoon on Chatham Island, and whose bones could still be found in the sand dunes.