Waters are warming in Southland at higher rates than elsewhere, with implications for fisheries and habitats.
Details of the issue have been captured in a November report from acting Environment Southland chief executive Rob Phillips.
Mr Phillips said sea-surface temperatures were increasing about 34% faster in New Zealand than the global average.
"While it may mean that the region can enjoy some warmer swimming temperatures, continued increases will have detrimental effects on a wide range of our coastal ecosystems including valued fisheries and habitats."
Council general manager science Karen Wilson provided further details, saying the ocean acted like a heat sponge, absorbing excess heat from the atmosphere and preventing dramatic rises in air and land temperatures. Warming waters fuelled extreme weather events and drove sea level rise.
"Rising temperatures can also disrupt ocean chemistry and nutrient cycles, altering major currents and threatening marine ecosystems."
That could lead to coral bleaching, potential food security issues and habitat changes for marine mammals, Dr Wilson said.
Ocean temperatures fluctuated naturally due to El Nino and La Nina cycles, changes in ocean currents and long-term climate patterns, she said.
"These natural influences combine with broader climate changes and drive the warming we are observing today."
The 0.4°C increase in average temperature also applied to the east coast of the South Island.
— LDR is local body journalism co-funded by RNZ and NZ On Air.
