Sea temperatures off the eastern South Island could warm by as much as 2C within the next century, a consortium of scientists said today.
The balmier seas would come on the back of more energetic wind patterns in the South Pacific which will sweep subtropical ocean currents further south.
Equatorial heat was already transferred into the Tasman Sea by the East Australian Current, which moves down the east coast of Australia to Tasmania and also across to New Zealand.
As atmospheric carbon dioxide levels climb and winds intensify, this current will become stronger and will move more heat southward, the scientists said.
The beginnings of this have been observed already.
The scientists base their findings on the study of microscopic marine plankton fossils called foraminifera, which were found in more than a dozen seabed sediment cores collected from the Tasman Sea and east of New Zealand.
The group, from GNS Science, Victoria University of Wellington, NIWA, and the University of Auckland, recently published their findings in the international science journal, Paleoceanography.
Analysis of the foraminifera enabled the scientists to reconstruct the last major global warm period about 125,000 years ago, when ocean temperatures were about 2C warmer than today.
"The ocean warming has already started, with temperatures off Tasmania having risen by 1.5 degrees Celsius in the past 70 years, which is more than twice the global average rate," said lead author Giuseppe Cortese of GNS Science.
The warming off Tasmania had been accompanied by an invasion of sub-tropical marine life, which had replaced subantarctic species.
Co-author Gavin Dunbar, of Victoria University of Wellington, said results were consistent with modern observations - namely a stronger East Australian Current that had extended 350km to the south to warm the seas off Tasmania.
"However, compared to modern observations, which cover seven decades, our data span thousands of years and hence provide insights into longer term change," Dr Dunbar said.
"A number of local studies have shown warming ocean temperatures in recent decades. However, this is the first region-wide study and it provides a much bigger geographical coverage."