Researchers found the New Zealand storm petrel egg on Little Barrier Island, off the northeast of the North Island, a year after the birds were tracked to breeding sites on the island.
Scientists took the opportunity when the female was off the nest to check the egg, which is expected to hatch in April, and record data, Xinhua reported citing a statement issued by New Zealand's Department of Conservation (DOC) statement.
"It was exciting to see the egg of a bird once thought to be extinct," Graeme Taylor, DOC principal science advisor, said in a statement leading the team.
"Measuring a mere 31 mm by 23 mm, the egg is white with a fine dusting of pink spots concentrated at one end," he said.
"The fact it has taken until 2014 for scientists to observe one of these tiny eggs reflects how much we still don't know about New Zealand's natural environment and particularly for marine species," he added.
The New Zealand storm petrel is a sparrow-sized bird that spends most of its life at sea.
Before 2003, when it was rediscovered, it was known only from three specimens collected in the 19th century and was presumed extinct for more than a century.
Little is known about the bird, including its population size.