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."
Dr Cortese said there was not a simple direct relationship between increased energy in ocean circulation patterns and sea temperatures around New Zealand.
"As temperatures increase off Australia's east coast, ocean circulation patterns change - heat transfer toward the North Island weakens, while heat transfer toward the South Island gets stronger.
"So the biggest amount of warming will be seen off the South Island's east coast."
In broad terms, this would mean sea temperatures off Southland would become more like today's sea temperatures in the Marlborough Sounds in 100 years' time. This would represent a significant change for marine ecosystems.
Dr Cortese said scientists were now turning their attention to how the changes would affect the marine food chain and ocean currents.
"From what we are seeing off the coast of Australia, both in recent observed trends and in our reconstruction from the past, such temperature changes are likely to have impacts on the whole marine ecosystem, and will ultimately impact on commercial fish stocks."
